Showing papers on "Partial melting published in 2007"

Special Paper: Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis

Abstract: Based on a compilation of published sources, rocks referred to as adakites show the following geochemical and isotopic characteristics: SiO2 ≥56 wt percent, Al2O3 ≥15 wt percent, MgO normally <3 wt percent, Mg number ≈0.5, Sr ≥400 ppm, Y ≤18 ppm, Yb ≤1.9 ppm, Ni ≥20 ppm, Cr ≥30 ppm, Sr/Y ≥20, La/Yb ≥20, and 87Sr/86Sr ≤0.7045. Rocks with such compositions have been interpreted to be the products of hybridization of felsic partial melts from subducting oceanic crust with the peridotitic mantle wedge during ascent and are not primary magmas. High Mg andesites have been interpreted to be related to adakites by partial melting of asthenospheric peridotite contaminated by slab melts. The case for these petrogenetic models for adakites and high Mg andesites is best made in the Archean, when higher mantle geotherms resulted in subducting slabs potentially reaching partial melting temperatures at shallow depths before dehydration rendered the slab infusible. In the Phanerozoic these conditions were likely only met under certain special tectonic conditions, such as subduction of young (≤25-m.y.-old) oceanic crust. Key adakitic geochemical signatures, such as low Y and Yb concentrations and high Sr/Y and La/Yb ratios, can be generated in normal asthenosphere-derived tholeiitic to calc-alkaline arc magmas by common upper plate crustal interaction and crystal fractionation processes and do not require slab melting. An assessment of several arc volcanic suites from around the world shows that most adakite-like compositions are generated in this way and do not reflect source processes. Similarly, rare adakite-like intrusive rocks associated with some porphyry Cu deposits are the evolved products of extensive crustal-level processing of calc-alkaline basalt-andesite-dacite-rhyolite series magmas. If slab melts contribute to such magmas, their geochemical signatures would have been obliterated or rendered ambiguous by subsequent extensive open-system processes. In Archean terranes, where adakitic and high Al tonalite-trondhjemite-granodiorite (TTG) magma series rocks are more common, porphyry Cu deposits are rare and, where found, are associated with normal calc-alkaline suites rather than adakites. The two different magma series are compositionally distinct in terms of several major and trace element parameters. Common upper plate magmatic processes such as melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC) affecting normal arc magmas can be demonstrated to explain the distinctive compositions of most adakite-like arc rocks, including high Mg andesites and especially those rare examples associated with porphyry Cu deposits. In contrast, slab melting can in most cases neither be proved nor disproved and is therefore unsatisfactory as a unique factor in porphyry Cu genesis.

A crystallizing dense magma ocean at the base of the Earth’s mantle

Abstract: If a stable layer of dense melt formed at the base of the mantle early in Earth's history, it would have undergone slow fractional crystallization and could provide an unsampled geochemical reservoir hosting a variety of incompatible geochemical species (most notably the missing budget of heat producing elements). The distribution of geochemical species in the Earth’s interior is largely controlled by fractional melting and crystallization processes that are intimately linked to the thermal state and evolution of the mantle. The existence of patches of dense partial melt at the base of the Earth’s mantle1, together with estimates of melting temperatures for deep mantle phases2 and the amount of cooling of the underlying core required to maintain a geodynamo throughout much of the Earth’s history3, suggest that more extensive deep melting occurred in the past. Here we show that a stable layer of dense melt formed at the base of the mantle early in the Earth’s history would have undergone slow fractional crystallization, and would be an ideal candidate for an unsampled geochemical reservoir hosting a variety of incompatible species (most notably the missing budget of heat-producing elements) for an initial basal magma ocean thickness of about 1,000 km. Differences in 142Nd/144Nd ratios between chondrites and terrestrial rocks4 can be explained by fractional crystallization with a decay timescale of the order of 1 Gyr. These combined constraints yield thermal evolution models in which radiogenic heat production and latent heat exchange prevent early cooling of the core and possibly delay the onset of the geodynamo to 3.4–4 Gyr ago5.

Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts

Abstract: We document compositions of minerals and melts from 3 GPa partial melting experiments on two carbonate-bearing natural lherzolite bulk compositions (PERC: MixKLB-1þ2 5 wt% CO2; PERC3: MixKLB-1þ1wt% CO2) and discuss the compositions of partial melts in relation to the genesis of alkalic to highly alkalic ocean island basalts (OIB). Near-solidus (PERC: 1075^11058C; PERC3: 10508C) carbonatitic partial melts with 510 wt% SiO2 and 40 wt% CO2 evolve continuously to carbonated silicate melts with 425 wt% SiO2 and525 wt%CO2 between 1325 and 13508C in the presence of residual olivine, orthopyroxene, clinopyroxene, and garnet. The first appearance of CO2-bearing silicate melt at 3 GPa is 1508C cooler than the solidus of CO2-free peridotite.The compositions of carbonated silicate partial melts between 1350 and 16008C vary in the range of 28^46 wt% SiO2, 1 6^0 5 wt% TiO2, 12^10 wt% FeO , and 19^29 wt% MgO for PERC, and 42^48 wt% SiO2, 1 9^0 5 wt%TiO2, 10 5^8 4 wt% FeO , and 15^26 wt%MgOforPERC3.TheCaO/Al2O3weight ratio ofsilicate melts ranges from 2 7 to 1 1 for PERC and from 1 7 to 1 0 for PERC3.The SiO2 contents of carbonated silicate melts in equilibrium with residual peridotite diminish significantly with increasing dissolved CO2 in the melt, whereas the CaO contents increase markedly. Equilibrium constants for Fe ^Mg exchange between carbonated silicate liquid and olivine span a range similar to those for CO2-free liquids at 3 GPa, but diminish slightly with increasing dissolved CO2 in the melt.The carbonated silicate partial melts of PERC3 at520% melting and partial melts of PERC at 15^33% melting have SiO2 andAl2O3 contents, and CaO/Al2O3 values, similar to those of melilititic to basanitic alkali OIB, but compared with the natural lavas they are more enriched in CaOand they lack the strong enrichments inTiO2 characteristic of highly alkalic OIB. If a primitive mantle source is assumed, theTiO2 contentsofalkalicOIB, combinedwith bulkperidotite/melt partition coefficients ofTiO2 determined in this study and in volatile-free studiesofperidotite partialmelting, canbe used to estimate that melilitites, nephelinites, and basanites from oceanic islands are produced from 0^6% partial melting.The SiO2 and CaOcontents of such small-degree partial melts of peridotite with small amounts of total CO2 can be estimated from the SiO2^CO2 and CaO^CO2 correlationsobserved inourhigher-degreepartialmeltingexperiments.These suggest that many compositional features of highly alkalic OIBmay be produced by 1^5% partial melting of a fertile peridotite source with 0 1^0 25 wt% CO2. Owing to very deep solidi of carbonated mantle lithologies, generationofcarbonated silicatemelts inOIBsource regions probably happens by reaction between peridotite and/or eclogite and migrating carbonatitic melts produced atgreaterdepths.

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Abstract: Partial melting of magnesite-bearing peridotites was studied at 6-10 GPa and 1300-1700°C. Experiments were performed in a multianvil apparatus using natural mineral mixes as starting material placed into olivine containers and sealed in Pt capsules. Partial melts originated within the peridotite layer, migrated outside the olivine container and formed pools of quenched melts along the wall of the Pt capsule. This allowed the analysis of even small melt fractions. Iron loss was not a problem, because the platinum near the olivine container became saturated in Fe as a result of the reaction Fe 2 SiO 4 Ol = Fe Fe-Pt alloy + FeSiO 3 Opx + O 2 . This reaction led to a gradual increase in oxygen fugacity within the capsules as expressed, for example, in high Fe 3+ in garnet. Carbonatitic to kimberlite-like melts were obtained that coexist with olivine + orthopyroxene + garnet ± clinopyroxene ± magnesite depending on P-T conditions. Kinetic experiments and a comparison of the chemistry of phases occasionally grown within the melt pools with those in the residual peridotite allowed us to conclude that the melts had approached equilibrium with peridotite. Melts in equilibrium with a magnesite-bearing garnet lherzolite are rich in CaO (20-25 wt %) at all pressures and show rather low MgO and SiO 2 contents (20 and 10 wt %, respectively). Melts in equilibrium with a magnesite-bearing garnet harzburgite are richer in SiO 2 and MgO. The contents of these oxides increase with temperature, whereas the CaO content becomes lower. Melts from magnesite-free experiments are richer in SiO 2 , but remain silicocarbonatitic. Partitioning of trace elements between melt and garnet was studied in several experiments at 6 and 10 GPa. The melts are very rich in incompatible elements, including large ion lithophile elements (LILE), Nb, Ta and light rare earth elements. Relative to the residual peridotite, the melts show no significant depletion in high field strength elements over LILE. We conclude from the major and trace element characteristics of our experimental melts that primitive kimberlites cannot be a direct product of single-stage melting of an asthenospheric mantle. They rather must be derived from a previously depleted and re-enriched mantle peridotite.

Discussions on the petrogenesis of granites

Abstract: As a major component of continental crust,granites have been served as the most important subject in geology.Based on advancements obtained during past decades,this paper provides a comprehensive overview about the issues related to granitic formation.As for genetic types,the classifications between I-,S-and A-type granites are sometimes difficult,especially for those of highly fractionated rocks.It is stated that the concentrations of zirconium in whole-rock and titanium in zircon can be used to provided information on the temperature of partial melting and magma crystallization,but the pressure under which the source partially melted is hard to estimate.The granites are mostly occurred in the subduction zones and post-orogenic extensional settings,where the inputs of volatile and heat resulted in crustal partial melting of orogenic roots,and then the formation of granites.The traditionally used geochemical diagrams for the tectonic discrimination could not get right answer in most cases.This paper also presents a concise summary about the recent achievements of granitic study in China.Finally,potential breakthroughs for the Mesozoic granites in eastern China are explored.

Archaean TTGs as sources of younger granitic magmas: melting of sodic metatonalites at 0.6–1.2 GPa

Abstract: Two natural, low K2O/Na2O, TTG tonalitic gneisses (one hornblende-bearing and the other biotite-bearing) were partially melted at 0.8–1.2 GPa (fluid-absent). The chief melting reactions involve the breakdown of the biotite and hornblende. The hornblende tonalite is slightly less fertile than the biotite tonalite, but melt volumes reach around 30% at 1,000°C. This contrasts with results of most previous work on more potassic TTGs, which generally showed much lower fertility, though commonly producing more potassic melts. Garnet is formed in biotite-bearing tonalitic protoliths at P > 0.8 GPa and at > 1.0 GPa in hornblende-bearing tonalitic protoliths. All fluid-absent experiments produced peraluminous granitic to granodioritic melts, typically with SiO2 > 70 wt.%. For the biotite tonalite, increasing T formed progressively more melt with progressively lower K2O/Na2O. However, the compositions of melts from the hornblende tonalite do not vary significantly with T. With increasing P, melts from the biotite tonalite become less potassic, due to the increasing thermal stability of biotite. For the hornblende tonalite, again there is no consistent trend. Fluid-absent melting of sodic TTGs produces melts with insufficient K2O to model the magmas that formed the voluminous, late, potassic granites that are common in Archaean terranes. Reconnaissance fluid-present experiments at 0.6 GPa imply that H2O-saturated partial melting of TTGs is also not a viable process for producing magmas that formed these granites. The protoliths for these must have been more potassic and less silicic. Nevertheless, at granulite-facies conditions, sodic TTGs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths. Upward abstraction of this melt would result in some LILE depletion of the terrane. Younger K-rich magmatism is unlikely to represent recycling of TTG crust on its own, and it seems most likely that evolved crustal rocks and/or highly enriched mantle must be involved.

Water solubility in aluminous orthopyroxene and the origin of Earth's asthenosphere.

Abstract: Plate tectonics is based on the concept of rigid lithosphere plates sliding on a mechanically weak asthenosphere. Many models assume that the weakness of the asthenosphere is related to the presence of small amounts of hydrous melts. However, the mechanism that may cause melting in the asthenosphere is not well understood. We show that the asthenosphere coincides with a zone where the water solubility in mantle minerals has a pronounced minimum. The minimum is due to a sharp decrease of water solubility in aluminous orthopyroxene with depth, whereas the water solubility in olivine continuously increases with pressure. Melting in the asthenosphere may therefore be related not to volatile enrichment but to a minimum in water solubility, which causes excess water to form a hydrous silicate melt.

The formation of SiO 2 -rich melts within the deep oceanic crust by hydrous partial melting of gabbros

Abstract: Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO2 is a key parameter for the discrimination between different processes: TiO2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO2 contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO2 is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes.

Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N: ODP Hole 1274A

Abstract: ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clin- opyroxene, olivine Mg# (up to 0.92) and spinel Cr# (~0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least re- acted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyrox- ene-rich lithology. Rare olivine-rich lherzolitic do- mains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clin- opyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt-rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equili- bration with residual harzburgites.

Geochemical character of serpentinites associated with high- to ultrahigh-pressure metamorphic rocks in the Alps, Cuba, and the Himalayas: Recycling of elements in subduction zones

Abstract: Serpentinites associated with eclogitic rocks were examined from three areas: the Alps, Cuba, and the Himalayas. Most serpentinites have low Al/Si and high concentrations of Ir-type platinum group elements (PGE) in bulk rock compositions, indicating that they are hydrated mantle peridotites. A few samples contain high Al/Si and low concentrations of Ir-type PGE, suggesting that they are ultramafic cumulates. Among the hydrated mantle peridotites, we identified two groups, primarily on the basis of Al/Si and Mg/Si ratios: forearc mantle serpentinites and hydrated abyssal peridotites. Forearc serpentinites occur in the Himalayas and along a major deformation zone in Cuba. All serpentinites in the Alps and most serpentinites in Cuba are hydrated abyssal peridotites. Himalayan serpentinites have low Al/Si and high Mg/Si ratios in bulk rock compositions, and high Cr in spinel; they were serpentinized by fluids released from the subducted Indian continent and enriched in fluid-mobile elements, and show high 87Sr/86Sr, up to 0.730, similar to the values of rocks of the subducted margin of the Indian continent. Although Himalayan serpentinites have a similar refractory geochemical signature as the Mariana forearc serpentinites, the former contain markedly high concentrations of fluid-mobile elements and high 87Sr/86Sr compared to the latter that were hydrated by subducted Pacific Ocean crust. The data indicate that the enrichment of fluid-mobile elements in forearc serpentinites depends on the composition of subducted slabs. Alpine serpentinites and most Cuban serpentinites show moderate Al/Si similar to abyssal peridotites. Hydration of peridotites near the seafloor is supported by micro-Raman spectra of earlier formed lizardite, high δ34S (+11 to +17‰) of sulphides, and elevated 87Sr/86Sr, ranging from 0.7037 to 0.7095. The data support the contribution of S and Sr from seawater and sediments. These serpentinites are not highly enriched in fluid-mobile elements because serpentinization occurred at a high water/rock ratio. Alkali elements are conspicuously unenriched in all serpentinites. This lack of alkali enrichment is explained by slab retention of alkalis. This is also consistent with the observation of relatively low alkali concentrations in volcanic front magmas, since partial melting related to the volcanic fronts is triggered by dehydration of serpentinites.

Effect of solid flow above a subducting slab on water distribution and melting at convergent plate boundaries

Abstract: [1] Hydrous fluids derived by dehydration of the downgoing slab at convergent plate boundaries are thought to provoke wet melting in the wedge above the downgoing plate. We have investigated the distribution of hydrous fluid and subsequent melt in the wedge using two-dimensional models that include solid mantle flow and associated temperature distributions along with buoyant fluid migration and melting. Solid mantle flow deflects hydrous fluid from their buoyant vertical migration through the wedge. Melting therefore does not occur directly above the region where hydrous fluids are released from the slab. A melting front develops where hydrous fluids first encounter mantle material hot enough to melt. Wet melting is influenced by solid flow through the advection of fertile mantle material into the wet melting region and the removal of depleted material. The region of maximum melting occurs where the maximum flux of water from slab mineral dehydration reaches the wet melting region. The extent of melting (F) and melt production rates increase with increasing convergence rate and grain size due to increased temperatures along the melting front and to increased fractions of water reaching the melting front, respectively. The position of isotherms above the wet solidus varies with increasing slab dip and thereby also influences F and melt production rates. Applying the understanding of wet melting from this study to geochemical studies of the Aleutians may help elucidate the processes influencing fluid migration and melt production in that region. Estimates of the timescale of fluid migration, seismic velocity variation, and attenuation are also investigated.

Cenozoic Crustal Evolution and Mantle Dynamics of Post-Collisional Magmatism in Western Anatolia

Abstract: Post-collisional magmatism in western Anatolia followed a continental collision event in the Early Eocene, and occurred in discrete pulses that appear to have propagated from north to south over time. The first episode occurred during the Eocene and Oligo-Miocene and was subalkaline in nature, producing medium- to high-K calc-alkaline granitoids and mafic to felsic volcanic rocks. Partial melting and assimilation–fractional crystallization of enriched subcontinental lithospheric mantle–derived magma(s) were important processes in the genesis and evolution of the parental magmas, which experienced decreasing subduction influence and increasing crustal contamination through the Early Eocene–Early Miocene. This magmatic episode coincided with continued regional compression and development of a thick orogenic crust, and was influenced by an influx of asthenospheric heat and melts provided by lithospheric slab break-off. Extensional tectonics replaced the regional compression by the Middle Miocene, following the initial collapse of the western Anatolian orogenic welt, and resulted in the development of metamorphic core complexes and horst-graben structures. The second main episode of magmatism occurred during the Middle Miocene (16–14 Ma) and produced mildly alkaline rocks that show a decreasing amount of crustal contamination and subduction influence through time. Although melting of a subduction-modified lithospheric mantle continued, an asthenospheric mantle–derived melt contribution played a major role in the generation of these mildly alkaline magmas. The inferred asthenospheric melt contribution was a result of delamination of the lowermost part of the lithospheric mantle and/or partial convective removal of the sub-continental lithospheric mantle (SCLM). The third episode of post-collisional magmatism started around ~12 Ma and continued through the Late Quaternary. The main melt source for this phase carried no subduction component and was generated by the decompressional melting of asthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in the Aegean extensional province. Lithospheric-scale extensional fault systems acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface. Post-collisional magmatism in western Anatolia thus displays compositionally distinct episodes controlled by slab break-off, lithospheric delamination, and asthenospheric upwelling and decompressional melting, reflecting the geodynamic evolution of the eastern Mediterranean region throughout the Cenozoic. These events and the associated processes in the mantle took place primarily in response to the plate tectonic evolution of the region and collectively constitute a time-progressive template for the mode and nature of the post-collisional magmatism common to most alpine-style orogenic belts.

Adakite-like porphyries from the southern Tibetan continental collision zones: evidence for slab melt metasomatism

Abstract: We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dyn/Ybn and Lan/Ybn. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial 87Sr/86Sr and 206Pb/204Pb ratios, and the highest 144Nd/143Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.

Zircon U-Pb ages and Hf isotope compositions of migmatite from the North Dabie terrane in China: constraints on partial melting

Abstract: Migmatite gneisses are widespread in the Dabie orogen, but their formation ages are poorly constrained. Eight samples of migmatite, including leucosome, melanosome, and banded gneiss, were selected for U-Pb dating and Hf isotope analysis. Most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains. Morphological and internal structure features suggest that their growth is associated with partial melting. According to the Hf isotope ratio relationships between metamorphic zircon and inherited cores, three formation mechanisms for metamorphic zircon can be determined, which are dissolution-reprecipitation of pre-existing zircon, breakdown of Zr-bearing phase other than zircon in a closed system and crystallization from externally derived Zr-bearing melt. Four samples contain magmatic zircon cores, yielding upper intercept U-Pb ages of 807 +/- 35-768 +/- 12 Ma suggesting that the protoliths of the migmatites are Neoproterozoic in age. The migmatite zircon yields weighted mean two-stage Hf model ages of 2513 +/- 97-894 +/- 54 Ma, indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation. The metamorphic zircons give U-Pb ages of 145 +/- 2-120 +/- 2 Ma. The oldest age indicates that partial melting commenced prior to 145 Ma, which also constrains the onset of extensional tectonism in this region to pre-145 Ma. The youngest age of 120 Ma was obtained from an undeformed granitic vein, indicating that deformation in this area was complete at this time. Two major episodes of partial melting were dated at 139 +/- 1 and 123 +/- 1Ma. The first episode of partial melting is obviously older than the timing of post-collision magmatism, corresponding to regional extension. The second episode of partial melting is coeval with the widespread post-collision magmatism, indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the Dabie orogen, which were possibly triggered by the uprising of the Cretaceous mid-Pacific superplume.

Partial Melting of Thickened or Delaminated Lower Crust in the Middle of Eastern China: Implications for Cu-Au Mineralization

Abstract: Field relations, isotope systematics, and plate tectonic reconstructions require that felsic adakites in the Yangtze Block and the Dabie Orogen, eastern China, were not derived from a subducting slab, despite the signature of a mantle component in the contemporaneous mafic adakite hosts of Cu‐Au deposits in the same areas. The apparently contradictory requirements are accounted for by (a) a deep crustal melting origin for barren adakites and (b) a crustal delamination origin, followed by ascent through lithospheric mantle, for adakites associated with mineralization. The crustal delamination process associated with the prospective porphyries duplicates the metallogenically essential aspects of the subduction environment. The importance of adakitic magmas in the genesis of porphyry‐style Cu‐Au deposits is affirmed by these findings, but the range of prospective tectonic environments is extended to include an important intraplate, postsubduction setting. The porphyries of eastern China demonstrate ...