Showing papers on "Metamorphism published in 2014"

The dilemma of the Jiaodong gold deposits: Are they unique?

Abstract: The ca. 126–120 Ma Au deposits of the Jiaodong Peninsula, eastern China, define the country's largest gold province with an overall endowment estimated as >3000 t Au. The vein and disseminated ores are hosted by NE- to NNE-trending brittle normal faults that parallel the margins of ca. 165–150 Ma, deeply emplaced, lower crustal melt granites. The deposits are sited along the faults for many tens of kilometers and the larger orebodies are associated with dilatational jogs. Country rocks to the granites are Precambrian high-grade metamorphic rocks located on both sides of a Triassic suture between the North and South China blocks. During early Mesozoic convergent deformation, the ore-hosting structures developed as ductile thrust faults that were subsequently reactivated during Early Cretaceous “Yanshanian” intracontinental extensional deformation and associated gold formation. Classification of the gold deposits remains problematic. Many features resemble those typical of orogenic Au including the linear structural distribution of the deposits, mineralization style, ore and alteration assemblages, and ore fluid chemistry. However, Phanerozoic orogenic Au deposits are formed by prograde metamorphism of accreted oceanic rocks in Cordilleran-style orogens. The Jiaodong deposits, in contrast, formed within two Precambrian blocks approximately 2 billion years after devolatilization of the country rocks, and thus require a model that involves alternative fluid and metal sources for the ores. A widespread suite of ca. 130–123 Ma granodiorites overlaps temporally with the ores, but shows a poor spatial association with the deposits. Furthermore, the deposit distribution and mineralization style is atypical of ores formed from nearby magmas. The ore concentration requires fluid focusing during some type of sub-crustal thermal event, which could be broadly related to a combination of coeval lithospheric thinning, asthenospheric upwelling, paleo-Pacific plate subduction, and seismicity along the continental-scale Tan-Lu fault. Possible ore genesis scenarios include those where ore fluids were produced directly by the metamorphism of oceanic lithosphere and overlying sediment on the subducting paleo-Pacific slab, or by devolatilization of an enriched mantle wedge above the slab. Both the sulfur and gold could be sourced from either the oceanic sediments or the serpentinized mantle. A better understanding of the architecture of the paleo-Pacific slab during Early Cretaceous below the eastern margin of China is essential to determination of the validity of possible models.

Multi-stage crustal growth and cratonization of the North China Craton

Abstract: The North China Craton (NCC) has a complicated evolutionary history with multi-stage crustal growth, recording nearly all important geological events in the early geotectonic history of the Earth. Our studies propose that the NCC can be divided into six micro-blocks with >∼3.0–3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts (GRB). The micro-blocks are also termed as high-grade regions (HGR) and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses, all of which underwent strong deformation and metamorphism of granulite- to high-grade amphibolite-facies. The micro-blocks are, in turn, from east to west, the Jiaoliao (JL), Qianhuai (QH), Ordos (ODS), Ji'ning (JN) and Alashan (ALS) blocks, and Xuchang (XCH) in the south. Recent studies led to a consensus that the basement of the NCC was composed of different blocks/terranes that were finally amalgamated to form a coherent craton at the end of Neoarchean. Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca. 2.9–2.7 and 2.6–2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC. Hafnium isotopic model ages range from ca. 3.8 to 2.5 Ga and mostly are in the range of 3.0–2.6 Ga with a peak at 2.82 Ga. Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered, with a dominant ca. 2.7 Ga magmatic zircon ages. Most of the ca. 2.7 Ga TTG gneisses underwent metamorphism in 2.6–2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks. Abundant ca. 2.6–2.5 Ga orthogneisses have Hf-in-zircon and Nd whole-rock model ages mostly around 2.9–2.7 Ga and some around 2.6–2.5 Ga, indicating the timing of protolith formation or extraction of the protolith magma was from the mantle. Therefore, it is suggested that the 2.6–2.5 Ga TTGs probably represent a coherent event of continental accretion and major reworking (crustal melting). As a distinct characteristic, nearly all GRBs in the NCC underwent amphibolite-facies metamorphism. Zircon U-Pb ages of metamorphosed GRB mafic rocks mainly show two peak ranges at ∼2.6–2.5 and 2.8–2.7 Ga. The mafic rocks are commonly believed to be derived from metabasalts, it is therefore possible that the ages represent the time of metamorphism. The tectonic settings of the GRBs are still a problem. Their geochemical characteristics are, respectively, similar to back-arc basins, rifts, island arcs or suggest imprints of mantle plumes. BIFs occur in all GRBs but also in the HGRs. This metallogenic specificity is quite different from all Phanerozoic geotectonic settings. The ∼2.5 Ga metamorphic-magmatic event is stronger than in most other cratons in the world. How to understand the geological significance of the 2.5 Ga event? The following points are emphasized: (1) nearly all old rocks >2.5 Ga underwent metamorphism at ∼2.52–2.5 Ga; (2) Archean basement rocks in the NCC experienced strong partial melting and migmatization; (3) granitoid rocks derived from partial melting include potassium granites, TTG granites and monzonites. These granitoids rocks intruded both the Archean greenstone belts and micro-blocks; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic-ultramafic dykes are also developed. Therefore, we suggest an assembly model that all micro-blocks in the NCC were welded together by late Archean greenstone belts at the end of the late Neoarchean. We also propose that the various micro-blocks were surrounded by small ocean basins, and the old continental crust and the oceanic crust were hotter than today. Subduction and collision were on much smaller scales as compared to the Phanerozoic plate tectonic regime, although the tectonic style and mechanisms were more or less similar. The formation of crustal melt granites is one of the processes of cratonization, inducing generation of stable upper and lower crustal layers. This process also generated an upper crust of more felsic composition and a lower crust of more mafic composition, due to molten residual materials and some underplated gabbros.

Early Paleozoic depositional environment and intraplate tectono-magmatism in the Cathaysia Block (South China): Evidence from stratigraphic, structural, geochemical and geochronological investigations

Abstract: The early Paleozoic geological evolution of the South China Craton composed of the Yangtze and Cathaysia Blocks has been the focus of long debate. The Cathaysia block has been central to the controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolite-facies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) the Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative eNd(t) values (−9.7 to −13.7) and two-stage eNd(t) model ages at ca.2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from the eroded Paleoproterozoic basement, and little or no mantle component was identified. During the Silurian, the Cathaysia block underwent strong folding, thrusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of the ductile sheared rocks revealed a fan-shape thrust pattern, with top-to-the southeast in the southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded 206Pb/238U ages of 435 ± 4 Ma, 424 ± 5 Ma, 428 ± 3 Ma and 427 ± 2 Ma. All the zircon eHf(t) values are negative (−6 to −9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that the Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of the early Paleozoic South China Fold Belt include the lack of early Paleozoic ophiolites and volcanic rocks, the absence of coeval HP-type blueschists, and the absence of mantle-derived juvenile magmatic rocks. Consequently, a subduction-collision-type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude that the South China Fold Belt was an intraplate orogen, and is possibly related to the global early Paleozoic continental assembly.

Himalayan Metamorphism and Its Tectonic Implications

Abstract: The Himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid- and deep crust that have fostered numerous models of how the crust responds to continental collisions. Recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one-time events. The pronounced metamorphic inversion across the Main Central Thrust reflects continuum thrusting between ca. 15 and 20 Ma, whereas exposure of ultrahigh-pressure rocks in northwestern massifs and syntaxis granulites reflects singular early (≥45 Ma) and late (≤10 Ma) exhumation events. Multiple mechanisms including wedge collapse and flow of melt-weakened midcrust are debated to explain pressure-temperature trajectories, patterns of thinning, and thermal overprinting. A geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co...

The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics

Abstract: In the early 1980s, evidence that crustal rocks had reached temperatures >1000 °C at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphic–metasomatic reworking of the lithospheric mantle was realized. In parallel, new developments in instrumentation, the expansion of in situ analysis of geological materials and increases in computing power opened up new fields of investigation. The robust quantification of pressure (P), temperature (T) and time (t) that followed these advances has provided reliable data to benchmark geodynamic models and to investigate secular change in the thermal state of the lithosphere as registered by metamorphism through time. As a result, the last 30 years have seen significant progress in our understanding of lithospheric evolution, particularly as it relates to Precambrian geodynamics. Eoarchean–Mesoarchean crust registers uniformly high T/P metamorphism that may reflect a stagnant lid regime. In contrast, two contrasting types of metamorphism, eclogite–high-pressure granulite metamorphism, with apparent thermal gradients of 350–750 °C/GPa, and granulite–ultrahigh temperature metamorphism, with apparent thermal gradients of 750–1500 °C/GPa, appeared in the Neoarchean rock record. The emergence of paired metamorphism is interpreted to register the onset of one-sided subduction, which introduced an asymmetric thermal structure at these developing convergent plate margins characterized by lower T/P in the subduction channel and higher T/P in the overriding plate. During the Paleoarchean to Paleoproterozoic the ambient mantle temperature was warmer than at present by ∼300–150 °C. Although the thermal history of Earth is only poorly constrained, it is likely that prior to ca. 3.0 Ga heating from radioactive decay would have exceeded surface heat loss, whereas since ca. 2.5 Ga secular cooling has dominated the thermal history of the Earth. The advent of paired metamorphism is consistent with other changes in the geological record during the Neoarchean that are best explained as the result of a transition from a stagnant lid to subduction and a global plate tectonics regime by ca. 2.5 Ga. This interpretation is supported by results from 2-D numerical experiments of oceanic subduction that demonstrate a change to one-sided subduction is plausible as upper mantle temperature declined to <200 °C warmer than at present during the late Neoarchean–Paleoproterozoic. This is the beginning of the Proterozoic plate tectonics regime. At 1.0 Ga the ambient mantle temperature was still ∼150–100 °C warmer than at present. Continued secular cooling caused a transition to cold subduction registered in the crustal record of metamorphism by the first appearance of blueschist and high to ultrahigh pressure metamorphism during the Neoproterozoic. Results of 2-D numerical experiments of continental collision demonstrate a transition from shallow to deep slab breakoff associated with stronger crust–mantle coupling that enabled continental subduction to mantle depths as upper mantle temperature declined to <100 °C warmer than at present during the late Proterozoic. This is the beginning of the modern plate tectonics regime.

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

Abstract: The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.

Thermal control on the modes of crustal thinning leading to mantle exhumation. Insights from the Cretaceous Pyrenean hot paleomargins.

Abstract: The pre-rift Mesozoic sequences of the Cretaceous passive margins fossilized in the North Pyrenean Zone (NPZ) are characterized by high temperature deformation in relation with thinning of the continental basement. Our compilation of chronological and geological data confirms a clear correlation between the distribution of the highest paleotemperatures in the pre-rift sedimentary cover and the loci of extreme crustal stretching. Geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic pre-rift sediments indicate an early attenuation of the rifted continental crust. This leads us to propose a mechanism of rifting involving boudinage of the continental crust. The lateral extraction of the Paleozoic basement occurred under the pre-rift cover that is decoupled on the Triassic clays and evaporates. The thermal conditions allowing coeval ductile deformation of the crust and of the pre-rift sediments leaded to the widening of basins devoid of large faulted blocks. We discuss the implications on the origin and significance of the granulites and the relations between flysch deposition and high temperature metamorphism of the pre-rift sediments. In the NPZ, Albian-Cenomanian flysch sequences were deposited synchronously with the syn-metamorphic ductile deformation of the pre-rift sequences. Since the base of the flysch deposits also recorded locally the high-temperature tectonic event, we propose an original mechanism for the evolution of the basins involving continuous deformation of the pre-rift metamorphic sediments. At the scale of the Pyrenean domain, our results suggest a strong lateral variability in the tectonic style of passive margins, in direct link with their thermic pattern

Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen

Abstract: The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.

Paleoproterozoic crustal evolution of the Hengshan–Wutai–Fuping region, North China Craton

Abstract: An arguable point regarding the Neoarchean and Paleoproterozoic crustal evolution of the North China Craton (NCC) is whether the tectonic setting in the central belt during the mid-Paleoproterozoic (2.35–2.0 Ga) was dominated by an extensional regime or an oceanic subduction–arc regime. A review of the mid-Paleoproterozoic magmatism and sedimentation for the Hengshan–Wutai–Fuping region suggests that a back-arc extension regime was dominant in this region. This conclusion is consistent with the observation that the 2.35–2.0 Ga magmatism shows a typical bimodal distribution where the mafic rocks mostly have arc affinities and the acidic rocks mainly comprise highly-fractioned calc-alkaline to alkaline (or A-type) granites, and that this magmatism was coeval with development of extensional basins characteristic of transgressive sequences with volcanic interlayers such as in the Hutuo Group. Although the final amalgamation of the NCC was believed to occur at ∼1.85 Ga, recent zircon U–Pb age dating for mica schist in the Wutai Group suggests a collisional event may have occurred at ∼1.95 Ga. The metamorphic ages of ∼1.85 Ga, obtained mostly from the high-grade rocks using the zircon U–Pb approach, most probably indicate uplifting and cooling of these high-grade terranes. This is because (i) phase modeling suggests that newly-grown zircon grains in high-grade rocks with a melt phase cannot date the age of peak pressure and temperature stages, but the age of melt crystallization in cooling stages; (ii) the metamorphic P–T paths with isobaric cooling under 6–7 kb for the Hengshan and Fuping granulites suggest their prolonged stay in the middle–lower crust; and (iii) the obtained metamorphic age data show a continuous distribution from 1.95 to 1.80 Ga. Thus, an alternative tectonic scenario for the Hengshan–Wutai–Fuping region involves: (i) formation of a proto-NCC at ∼2.5 Ga; (ii) back-arc extension during 2.35–2.0 Ga resulting in bimodal magmatism and sedimentation in rifting basins on an Archean basement; (iii) a crustal thickening event in the extended region resulting in a kyanite-type metamorphism at ∼1.95 Ga, and (iv) uplifting and cooling of the thickened crust from 1.93 to 1.80 Ga.

Partial melting of deeply subducted eclogite from the Sulu orogen in China

Abstract: We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U-Pb dating and petrological analyses, that partial melting occurred at 228-219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust.

Low-pressure - high-temperature metamorphism during extension in a Jurassic magmatic arc, Central Pontides, Turkey

Abstract: Magmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature–low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high‐T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high‐T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K‐feldspar + plagioclase + biotite + cordierite ± sillimanite ± garnet. Mineral equilibria give peak metamorphic conditions of 4 ± 1 kbar and 720 ± 40 °C. Zircon U–Pb and biotite Ar–Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300 °C at c. 162 Ma, when they were intruded by shallow‐level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.

Chemical and Isotopic Cycling in Subduction Zones

Abstract: Subduction zones are avenues for the delivery of crustal, atmospheric, and oceanic (including organic) components to the mantle and understanding of subduction zone chemical and isotopic cycling is central to many models of crust–mantle–atmosphere evolution During subduction, diagenetic and metamorphic reactions and related geochemical effects can profoundly influence the element inventory and isotopic composition of subducting slabs to depths beneath volcanic arcs and as they enter the deeper mantle Physical juxtaposition of chemically disparate rocks and the generation and mobility of various fluids lead to myriad metasomatic effects in subduction zones Larger scale manifestations of such processes include the mass transfer that leads to arc magmatism and convergent margin volatile cycling Subduction zone metasomatism is initiated at very shallow levels, as oceanic slabs entering trenches bend and are infiltrated by seawater, and as sedimentary sections experience physical compaction, fluid expulsion, and diagenetic alteration Studies of forearc fluid geochemistry track this shallow-level metasomatic alteration, and high- and ultrahigh-pressure metamorphic rock suites provide records of fluid generation and flow, and related metasomatism, to depths exceeding those beneath volcanic fronts Subduction zone processes act as a geochemical filter, altering the compositions of deeply subducting rocks and generating outputs such as arc magmas (and associated volcanic gases) and chemically and isotopically modified rocks The latter enter the deeper mantle and influence its long-term geochemical evolution

Geochronological, geochemical, and Nd-Hf isotopic studies of the Qinling Complex, central China: Implications for the evolutionary history of the North Qinling Orogenic Belt

Abstract: The Qinling Complex of central China is thought to be the oldest rock unit and the inner core of the North Qinling Orogenic Belt (NQOB). Therefore, the Qinling Complex is the key to understanding the pre-Paleozoic evolution of the NQOB. The complex, which consists of metagraywackes and marbles, underwent regional amphibolite-facies metamorphism. In this study, we constrained the formation age of the Qinling Complex to the period between the late Mesoproterozoic and the early Neoproterozoic (ca. 1062–962 Ma), rather than the Paleoproterozoic as previously thought. The LA-ICP-MS data show two major metamorphic ages (ca. 499 and ca. 420–400 Ma) for the Qinling Complex. The former age is consistent with the peak metamorphic age of the high- and ultra-high pressure (HP-UHP) rocks in the Qinling Complex, indicating that both the HP-UHP rocks and their country rocks experienced intensive regional metamorphism during the Ordovician. The latter age may constrain the time of partial melting in the NQOB between the late Silurian and early Devonian. The Qinling Complex is mostly affiliated with subduction–accretion processes along an active continental margin, and should contain detritus deposited in a forearc basin. The available data indicate that the NQOB was an independent micro-continent at least prior to the Neoproterozoic, and included a portion of the Grenville orogenic belt during the period of 1.2–0.8 Ga. The NQOB has experienced a unique geological history, which is obviously different from that of the North China Craton (NCC) and the Yangtze Craton during the Precambrian. The Neoproterozoic granitoids that intruded the Qinling Complex can be interpreted as the products of assembly of the supercontinent Rodinia. The NQOB was separated from Rodinia at ca. 830–740 Ma. Subsequently, the NQOB moved closer to the northern margin of the NCC, and the initial accretion or collision with the NCC occurred from the late Cambrian to the early Ordovician.

Zoned Monazite and Zircon as Monitors for the Thermal History of Granulite Terranes: an Example from the Central Indian Tectonic Zone

Abstract: The growth and dissolution behaviour of detrital, metamorphic and magmatic monazite and zircon during granulite-facies anatexis in pelitic and psammo-pelitic granulites and in garnetiferous granite from the southern margin of the Central Indian Tectonic Zone (CITZ) have been investigated using reconstructed metamorphic reaction history, monazite electron microprobe dating and sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon geochronology. Whereas the pelitic granulites record medium-pressure granulite-facies metamorphism (BM1 stage), the psammo-pelitic granulite reached ultrahigh temperatures (TMax > 880°C at 8·7 kbar). The meta-psammite additionally records two stages of granulite-facies recrystallization (BM2 and BM3). Irrespective of variations in the bulk-rock compositions and peak metamorphic conditions, monazite is highly reactive during the BM1 event, producing complex, chemically zoned crystals. Textural, compositional and chemical ages of these grains indicate the stability of six compositional domains (CD1 to CD6 in the paragenetic sequence), of which CD1 represents pre-metamorphic detrital cores of Paleoproterozoic age. CD2 and CD3 (combined mean age of 1612 ± 14 Ma) mark two stages of recrystallization of detrital monazite cores during prograde events. Rims of CD4 monazite (ages between 1615 ± 14 and 1586 ± 14 Ma) on partially to completely equilibrated cores indicate melt crystallization at, or immediately following, peak BM1P metamorphism. CD5 monazite (age of 1574 ± 7 Ma) is restricted to the psammo-pelitic granulites, and marks final melt crystallization at the solidus during post-peak cooling (BM1R stage, where R represents retrograde metamorphism). The metamorphic rim of CD6 monazite (age of 1539 ± 24 Ma) around partially resorbed CD5 domains is linked to the decomposition of BM1 garnet during the terminal hydration event as part of a granulite-facies recrystallization event. Compositionally homogeneous monazite and rims of chemically zoned monazite grains in granite together record a magmatic crystallization age of 1604 ± 9 Ma. SHRIMP U–Pb zircon dating of the psammo-pelitic granulite and garnetiferous granite indicates detrital or inherited cores of Paleo- to Neoarchean age (3584 ± 3 to 2530 ± 3 Ma), which have been variously recrystallized and overgrown by new zircon: (1) at 1658 ± 12 Ma; (2) between 1595 ± 5 and 1590 ± 6 Ma; (3) at 1574 ± 9 Ma. These zircon dates are correlated with the timing of the following: (1) the protoliths of precursor sediments of the metasedimentary granulites, deposited between 2530 and 1658 Ma; (2) a short-lived high-grade event ∼65–70 Myr before the culmination of the BM1 granulite-facies event; (3) a high-T anatectic event, corresponding to the peak BM1P metamorphism at TMax > 900°C; (4) final crystallization of anatectic melt at the solidus (cf. BM1R metamorphic stage). These chronological constraints from monazite and zircon, when integrated with the metamorphic reaction history and published geochronological data, allow recognition of three episodes of granulite-facies metamorphism in the CITZ at 1658 Ma (pre-BM1 event), between 1612 and 1574 Ma (BM1 event), and between 1572 and 1539 Ma (combined BM2 and BM3 events), as part of a latest Paleoproterozoic to Early Mesoproterozoic orogenic event. This orogeny is linked to the growth of the Proto-Greater Indian Landmass.

Mesozoic gold metallogenic system of the Jiaodong gold province,eastern China

Abstract: Jiaodong Peninsula is the most important gold concentration area of China,more than 150 gold deposits within it have been found and the proven gold reserves add up to 4000 tons. The amount of gold deposits and gold resource are huge,gold occurrence and mineralization type vary with the widely distributed gold deposits,however,the metallogenic geodynamics background,ore-host rock environment,gold occurrence conditions and metallogenic characteristics are identical as a whole:( 1) Jiaodong area is an endogenic hydrothermal gold concentration area consisting of Precambrian base rocks and ultra-high pressure( UHP) metamorphic rocks,tectonism and magmatism happened frequently in Mesozoic,130 ~ 110 Ma gold metallogenic events occurred 2000 Myr or so later than the regional metamorphism;( 2) regional gold metallogenic system formed in the Early Cretaceous continental-margin extension tectonic background,large-scale gold metallogenic events happened in the process of regional NW extension changing to NE extension followed by the NEE compression,which corresponded to the lithosphere reduction in East China,North China craton destruction and the peak of continental rifting;( 3) gold deposits clustered around the NNE Linglong,Queshan and Kunyushan metamorphic core complexes,mainly along the regional NE-NNE detachment faults developing along the contact zone of Precambrian metamorphic rocks and Mesozoic granites;( 4) the ore-controlling fault belts went through the early ductile-brittle deformation and late brittle deformation structure superposition,extended in smooth-out waveforms in 3D space and controlled the lateral trending and subsection enrichment of gold orebodies;( 5) the mineralization types mainly include clastic altered( breccia) rock type,( sulfide-) quartz vein type and compound vein-belt type,textures like the crush texture,crystalline-granular texture,interstitial texture and structures like the disseminated structure,vein structure,massive structure,crumby structure are abundant in the ore rocks,which indicates the oreforming environment changed from ductile-brittle conditions to brittle conditions;( 6) metallic minerals mainly include pyrite,chalcopyrite,galena and sphalerite,non-metallic minerals mainly include quartz,sericite,potash-feldspar and calcite; gold minerals mainly include the electrum,natural gold and a small amount of kustelite,which mainly occur in the fractures of pyrites and quartz in the form of visible gold,lesser in crystal gap and as inclusions; hydrothermal alteration types mainly include pyritization,silicification,sericitization and carbonatation; ore-forming elements mainly consist of the Au-Ag(-Cu-Pb-Zn) assemblage; the alteration and mineralization assemblage mentioned above show the characterisics of mesothermal-epithermal assemblage;( 7) ore-forming fluids came from both the crust and mantle and are mainly the crust source metamorphic fluids; metallogenic materials derived from the Precambrian metamorphic basement rock mass which reactivated in the Mesozoic,mingling with a small amount of the shallow crustal and mantle components. The consistency of regional metallogenic characteristics indicate that the Early Cretaceous large-scale gold metallogenesis in Jiaodong gold concentration area is controlled by the uniform geological events,and the gold metallogenesis belongs to an epigeneticmesothermal-epithermal hydrothermal vein gold metallogenic system. These gold deposits have obvious characteristic of spatio-temporal cluster distribution and lie mainly along the contact zones of different lithofacies around three metamorphic core complexes. From west to east,the gold mineralization age changes from older to newer. Therefore,three gold subsystems can be divided,which are the altered rock-quartz vein type in Jiaobei Uplift,the sulfide-quartz vein type in Sulu UHP metamorphic belt and the altered breccia type in north margin of Jiaolai Basin. The mineralization style changes from disseminated-veinlet,veinlet-stockwork and quartz vein type,to sulfide-quartz vein type,to altered breccia type. The texture and structure of ores are characterized by veinlet-disseminated structure dominated,to band structure and comb structure,and then to breccia structure,indicating mineralization occurred respectively in brittle-ductile transformation zone( ca. 15 km deep),a brittle extension-shear fault zone,and a brittle breccia zone( ca. 5km deep).The decrease of the size and strength of alteration and mineralization,and the increase of shallow crustal components in metallogenic materials,may be related to the deposits' location which is more and more far away from the source area. The ore-forming P-T conditions gradually decreased and the meteoric water and / or basin brine ratio in the ore-forming fluid gradually increased,respectively,which corresponds to the shallower metallogenic depth and more and more extended trending mineralization tectonic environment. All the regional regular changes of metallogenic characteristics reflect a crustal continuum metallogenic in different vertical depth of crust,between the detachment fault ductile-brittle transition zone and the brittle breccia zone. Mesozoic gold metallogenic system of the Jiaodong gold province is distinct from typical"intrusion-related gold deposit","orogenic gold deposit"or other known gold deposit type around the globe,and can't be classified into the known metallogenic model. To reasonably explain the unique geodynamic background,environment of ore-host rock and mineralization characteristics,we put forward the new understanding of the"Jiaodong type"gold deposit and metallogenic model. We conclude that the rollback of ancient Pacific Izanagi subduction plate may be the main drive mechanism leading to large-scale revitalizing of the metallogenic materials in regional Precambrian metamorphic basement rock mass,and ore-forming fluids mainly came from metamorphic dehydration of the subducting plate. Gold is mainly in Au( HS)2-complex and transported along detachment fault system in ore fluids. The tectonic space increases sharply as well as the metallogenic temperature and pressure decreases suddenly around the brittle-ductile transformation zone of detachment fault system and brittle breccia zone. Therefore,CO2 and H2S lossing from the ore fluids and sulfidation leads to a stability decreasing of Au( HS)2-and other gold complexes,and subsequent large-scale gold precipitation.

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Abstract: Blueschist facies terranes in the Variscan Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Cean Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica–Tui Complex (NW Iberian Massif). The Cean Unit is a highly condensed metamorphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane–chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composition that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P–T space through pseudosection approach and is characterised by H2O-undersaturated prograde evolution induced by the crystallisation of lawsonite. Peak conditions in the blueschist/LT-eclogite facies have been constrained at ca. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly isothermal decompression from the lawsonite-bearing fields to fields with stable albite at P ≈ 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite–greenschist facies overprint at P < 0.8 GPa and T ≈ 440–480 °C. The preservation of the lawsonite crystal shape despite complete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blueschist host rock were not affected by penetrative deformation during exhumation. 40Ar/39Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ca. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to represent some point relatively close to peak conditions at the onset of the isothermal decompression. 40Ar/39Ar dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre–Cean detachment, at the base of the Cean Unit, yields an age of ca. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21–45, 2005) support the equivalence of the Cean Unit and the Upper Unit of Ile de Groix along the Ibero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ca. 360–370 Ma, that is inferred to represent the Late Devonian–Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia.

Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland)

Abstract: At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.