Showing papers on "Basalt published in 2018"

The Central Atlantic Magmatic Province (CAMP): A Review

Abstract: The Central Atlantic magmatic province (CAMP) consists of basic rocks emplaced as shallow intrusions and erupted in large lava flow fields over a land surface area in excess of 10 million km2 on the supercontinent Pangaea at about 201 Ma. The peak activity of the CAMP straddled the Triassic-Jurassic boundary and probably lasted less than 1 million years, while late activity went on for several Ma more into the Sinemurian. Emission of carbon and sulfur from the CAMP magmas and from intruded sediments probably caused extinctions at the end-Triassic. Intrusive rocks are represented by isolated dykes up to 800 km-long, by dense dyke swarms and by extremely voluminous sills and a few layered intrusions. Lava fields were erupted as short-lived pulses and can be traced over distances of several hundred km within sedimentary basins. They consist of either compound or simple pahoehoe flows. Globally, the intrusive and effusive rocks are estimated to represent an original magmatic volume of at least 3 million km3. Herein we subdivide the CAMP basalts for the first time into six main geochemical groups, five represented by low-Ti and one by high-Ti rocks. Except for one low-Ti group, which is ubiquitous throughout the entire province, all other groups occur in relatively restricted areas and their compositions probably reflect contamination from the local continental lithosphere. Major and trace elements and Sr-Nd-Pb-Os isotopic compositions indicate that the basaltic magmas had an enriched composition compared to Mid-Ocean Ridge basalts and different from Atlantic Ocean Island basalts. The enriched composition of CAMP basalts is only in part attributable to crustal contamination. It also probably requires subducted upper and lower continental crust material that enriched the shallow upper mantle from which CAMP basalts were generated. A contribution from a deep mantle-plume is not required by geochemical and thermometric data, but it remains unclear what other possible heat source caused mantle melting on the scale required to form CAMP.

Rapid eruption of the Columbia River flood basalt and correlation with the mid-Miocene climate optimum.

Abstract: Flood basalts, the largest volcanic events in Earth history, are thought to drive global environmental change because they can emit large volumes of CO2 and SO2 over short geologic time scales. Eruption of the Columbia River Basalt Group (CRBG) has been linked to elevated atmospheric CO2 and global warming during the mid-Miocene climate optimum (MMCO) ~16 million years (Ma) ago. However, a causative relationship between volcanism and warming remains speculative, as the timing and tempo of CRBG eruptions is not well known. We use U-Pb geochronology on zircon-bearing volcanic ash beds intercalated within the basalt stratigraphy to build a high-resolution CRBG eruption record. Our data set shows that more than 95% of the CRBG erupted between 16.7 and 15.9 Ma, twice as fast as previous estimates. By suggesting a recalibration of the geomagnetic polarity time scale, these data indicate that the onset of flood volcanism is nearly contemporaneous with that of the MMCO.

Evidence for extremely rapid magma ocean crystallization and crust formation on Mars

Abstract: The formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood. The mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium-to-lead (U-Pb) isotopic decay system and is resistant to subsequent alteration. Moreover, given the high concentration of hafnium in zircon, the lutetium-to-hafnium (176Lu-176Hf) isotopic decay system can be used to determine the nature and formation timescale of its source reservoir1-3. Ancient igneous zircons with crystallization ages of around 4,430 million years (Myr) have been reported in Martian meteorites that are believed to represent regolith breccias from the southern highlands of Mars4,5. These zircons are present in evolved lithologies interpreted to reflect re-melted primary Martian crust 4 , thereby potentially providing insight into early crustal evolution on Mars. Here, we report concomitant high-precision U-Pb ages and Hf-isotope compositions of ancient zircons from the NWA 7034 Martian regolith breccia. Seven zircons with mostly concordant U-Pb ages define 207Pb/206Pb dates ranging from 4,476.3 ± 0.9 Myr ago to 4,429.7 ± 1.0 Myr ago, including the oldest directly dated material from Mars. All zircons record unradiogenic initial Hf-isotope compositions inherited from an enriched, andesitic-like crust extracted from a primitive mantle no later than 4,547 Myr ago. Thus, a primordial crust existed on Mars by this time and survived for around 100 Myr before it was reworked, possibly by impacts4,5, to produce magmas from which the zircons crystallized. Given that formation of a stable primordial crust is the end product of planetary differentiation, our data require that the accretion, core formation and magma ocean crystallization on Mars were completed less than 20 Myr after the formation of the Solar System. These timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns, resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust6,7.

CaSiO 3 perovskite in diamond indicates the recycling of oceanic crust into the lower mantle

Abstract: The composition of natural calcium silicate perovskite, the fourth most abundant mineral in the Earth, found within a diamond indicates an origin from oceanic crust subducted deeper than 700 kilometres into the Earth’s mantle. Within the Earth's transition zone and lower mantle, the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3) is thought to be the main host of calcium, as well as the heat-producing elements potassium, uranium and thorium. Despite being considered as the fourth most abundant mineral in the Earth, it has never been found in nature. Fabrizio Nestola and co-authors document the perovskite-structured polymorph of CaSiO3 included within a diamond from Cullinan kimberlite mined in South Africa. The authors conclude that the bulk composition of material within the diamond is consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle, providing additional evidence for the recycling of oceanic crust and carbon from the surface to lower-mantle depths. Laboratory experiments and seismology data have created a clear theoretical picture of the most abundant minerals that comprise the deeper parts of the Earth’s mantle. Discoveries of some of these minerals in ‘super-deep’ diamonds—formed between two hundred and about one thousand kilometres into the lower mantle—have confirmed part of this picture1,2,3,4,5. A notable exception is the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3). This mineral—expected to be the fourth most abundant in the Earth—has not previously been found in nature. Being the dominant host for calcium and, owing to its accommodating crystal structure, the major sink for heat-producing elements (potassium, uranium and thorium) in the transition zone and lower mantle, it is critical to establish its presence. Here we report the discovery of the perovskite-structured polymorph of CaSiO3 in a diamond from South African Cullinan kimberlite. The mineral is intergrown with about six per cent calcium titanate (CaTiO3). The titanium-rich composition of this inclusion indicates a bulk composition consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle. The relatively ‘heavy’ carbon isotopic composition of the surrounding diamond, together with the pristine high-pressure CaSiO3 structure, provides evidence for the recycling of oceanic crust and surficial carbon to lower-mantle depths.

Generation of Cenozoic intraplate basalts in the big mantle wedge under eastern Asia

Abstract: The roles of subduction of the Pacific plate and the big mantle wedge (BMW) in the evolution of east Asian continental margin have attracted lots of attention in past years. This paper reviews recent progresses regarding the composition and chemical heterogeneity of the BMW beneath eastern Asia and geochemistry of Cenozoic basalts in the region, with attempts to put forward a general model accounting for the generation of intraplate magma in a BMW system. Some key points of this review are summarized in the following. (1) Cenozoic basalts from eastern China are interpreted as a mixture of high-Si melts and low-Si melts. Wherever they are from, northeast, north or south China, Cenozoic basalts share a common low-Si basalt endmember, which is characterized by high alkali, Fe 2 O 3 T and TiO2 contents, HIMU-like trace element composition and relatively low 206Pb/204Pb compared to classic HIMU basalts. Their Nd-Hf isotopic compositions resemble that of Pacific Mantle domain and their source is composed of carbonated eclogites and peridotites. The high-Si basalt endmember is characterized by low alkali, Fe 2 O 3 T and TiO2 contents, Indian Mantle-type Pb-Nd-Hf isotopic compositions, and a predominant garnet pyroxenitic source. High-Si basalts show isotopic provinciality, with those from North China and South China displaying EM1-type and EM2-type components, respectively, while basalts from Northeast China containing both EM1- and EM2-type components. (2) The source of Cenozoic basalts from eastern China contains abundant recycled materials, including oceanic crust and lithospheric mantle components as well as carbonate sediments and water. According to their spatial distribution and deep seismic tomography, it is inferred that the recycled components are mostly from stagnant slabs in the mantle transition zone, whereas EM1 and EM2 components are from the shallow mantle. (3) Comparison of solidi of garnet pyroxenite, carbonated eclogite and peridotite with regional geotherm constrains the initial melting depth of high-Si and low-Si basalts at and ~300 km, respectively. It is suggested that the BMW under eastern Asia is vertically heterogeneous, with the upper part containing EM1 and EM2 components and isotopically resembling the Indian mantle domain, whereas the lower part containing components derived from the Pacific mantle domain. Contents of H2O and CO2 decrease gradually from bottom to top of the BMW. (4) Melting of the BMW to generate Cenozoic intraplate basalts is triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.

Hainan mantle plume produced late Cenozoic basaltic rocks in Thailand, Southeast Asia.

Abstract: Intraplate volcanism initiated shortly after the cessation of Cenozoic seafloor spreading in the South China Sea (SCS) region, but the full extent of its influence on the Indochina block has not been well constrained. Here we present major and trace element data and Sr-Nd-Pb-Hf isotope ratios of late Cenozoic basaltic lavas from the Khorat plateau and some volcanic centers in the Paleozoic Sukhothai arc terrane in Thailand. These volcanic rocks are mainly trachybasalts and basaltic trachyandesites. Trace element patterns and Sr-Nd-Pb-Hf isotopic compositions show that these alkaline volcanic lavas exhibit oceanic island basalt (OIB)-like characteristics with enrichments in both large-ion lithophile elements (LILE) and high field strength elements (HFSEs). Their mantle source is a mixture between a depleted Indian MORB-type mantle and an enriched mantle type 2 (EMII). We suggest that the post-spreading intraplate volcanism in the SCS region was induced by a Hainan mantle plume which spread westwards to the Paleozoic Sukhothai arc terrane.

Recycled ancient ghost carbonate in the Pitcairn mantle plume

Abstract: The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate-silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

Partial melting of lower oceanic crust gabbro: constraints from poikilitic clinopyroxene primocrysts

Abstract: Successive magma batches underplate, ascend, stall and erupt along spreading ridges, building the oceanic crust It is therefore important to understand the processes and conditions under which magma differentiates at mid ocean ridges Although fractional crystallization is considered to be the dominant mechanism for magma differentiation, open-system igneous complexes also experience Melting-Assimilation-Storage-Hybridization (MASH, Hildreth and Moorbath, 1988) processes Here, we examine crystal-scale records of partial melting in lower crustal gabbroic cumulates from the slow-spreading Atlantic oceanic ridge (Kane Megamullion; collected with Jason ROV) and the fast-spreading East Pacific Rise (Hess Deep; IODP expedition 345) Clinopyroxene oikocrysts in these gabbros preserve marked intra-crystal geochemical variations that point to crystallization-dissolution episodes in the gabbro eutectic assemblage Kane Megamullion and Hess Deep clinopyroxene core1 primocrysts and their plagioclase inclusions indicate crystallization from high temperature basalt (>1,160 and >1,200°C, respectively), close to clinopyroxene saturation temperature (<50% and <25% crystallization) Step-like compatible Cr (and co-varying Al) and incompatible Ti, Zr, Y and rare earth elements (REE) decrease from anhedral core1 to overgrown core2, while Mg# and Sr/Sr* ratios increase We show that partial resorption textures and geochemical zoning result from partial melting of REE-poor lower oceanic crust gabbroic cumulate (protolith) following intrusion by hot primitive mantle-derived melt, and subsequent overgrowth crystallization (refertilization) from a hybrid melt In addition, toward the outer rims of crystals, Ti, Zr, Y and the REE strongly increase and Al, Cr, Mg#, Eu/Eu*, and Sr/Sr* decrease, suggesting crystallization either from late-stage percolating relatively differentiated melt or from in situ trapped melt Intrusion of primitive hot reactive melt and percolation of interstitial differentiated melt are two distinct MASH processes in the lower oceanic crust They are potentially fundamental mechanisms for generating the wide compositional variation observed in mid-ocean ridge basalts We furthermore propose that such processes operate at both slow- and fast-spreading ocean ridges Thermal numerical modeling shows that the degree of lower crustal partial melting at slow-spreading ridges can locally increase up to 50%, but the overall crustal melt volume is low (less than ca 5% of total mantle-derived and crustal melts; ca 20% in fast-spreading ridges)

Thermal effects of pyroxenites on mantle melting below mid-ocean ridges

Abstract: After travelling in Earth’s interior for up to billions of years, recycled material once injected at subduction zones can reach a subridge melting region as pyroxenite dispersed in the host peridotitic mantle. Here we study genetically related crustal basalts and mantle peridotites sampled along an uplifted lithospheric section created at a segment of the Mid-Atlantic Ridge through a time interval of 26 million years. The arrival of low-solidus material into the melting region forces the elemental and isotopic imprint of the residual peridotites and of the basalts to diverge with time. We show that a pyroxenite-bearing source entering the subridge melting region induces undercooling of the host peridotitic mantle, due to subtraction of latent heat by melting of the low-T-solidus pyroxenite. Mantle undercooling, in turn, lowers the thermal boundary layer, leading to a deeper cessation of melting. A consequence is to decrease the total amount of extracted melt, and hence the magmatic crustal thickness. The degree of melting undergone by a homogeneous peridotitic mantle is higher than the degree of melting of the same peridotite but veined by pyroxenites. This effect, thermodynamically predicted for a marble-cake-type peridotite–pyroxenite mixed source, implies incomplete homogenization of recycled material in the convective mantle. Pyroxenite—recycled, subducted material—beneath mid-ocean ridges cools the mantle, suppressing melt extraction and crust formation, according to geochemical analyses of samples taken from the Mid-Atlantic Ridge.

Boron Isotopes in the Ocean Floor Realm and the Mantle

Abstract: This chapter reviews the boron isotopic composition of the ocean floor, including pristine igneous oceanic crust such as mid-ocean ridge basalts and ocean island basalts and their implications for the B isotopic composition of the mantle. The chapter further discusses the B isotopic effects of assimilation of altered crustal materials in mantle-derived magmas. The systematics of seawater alteration on oceanic rocks are discussed, including sediments, igneous crust and serpentinization of ultramafic rocks and the respective marine hydrothermal vent fluids. The chapter concludes with a discussion of the secular evolution of the B isotopic composition of seawater.

Timing and mechanism of the rise of the Shillong Plateau in the Himalayan foreland

Abstract: Island arcs provide insights into the origin and recycling of continental crust, but questions remain concerning source metasomatism, the depth of differentiation, the potential role of amphibole fractionation, and the time scales involved. Basaltic andesites on Batan Island (Philippines) contain ≥150 Ma peridotite fragments encased in hornblendite and gabbroic rinds produced via melt-rock reaction. The peridotites have some of the lightest δ7Li and δ57Fe values yet measured in mantle rocks. These values are too fractionated to have been created solely by equilibrium partial melting and thus require a combination of melt depletion and slab fluid addition and may be derived from diffusion-modified melt channel wall rocks. Stable isotope signals are easily modified by diffusive equilibration between peridotite and host magma, so the preservation of light δ7Li and δ57Fe here suggests magma ascent rates of ∼10 km yr−1. We show that melt–wall rock reactions at ∼25–30 km depth led to the crystallization of amphibole (± plagioclase) followed by gabbroic fractionation at ∼7 km depth. The former provides a location and mechanism for the “cryptic” amphibole fractionation observed in these and perhaps many other arc lavas and may obviate the requirement for delamination of cumulates.

Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source.

Abstract: Neodymium-142 isotope data from young Reunion Island volcanic rocks reflect the effects of geological processes that occurred more than four billion years ago, showing that the deep mantle may preserve geochemical signatures of the primordial Earth. Certain volcanic hotspots are thought to sample sections of Earth's deep interior that were formed more than two billion years ago, with some having witnessed differentiation events—the separation of planetary materials into chemically distinct regions, such as the core and mantle—that occurred within the first 50 million years of Earth's history. Bradley Peters and colleagues report neodymium isotopic data from Reunion Island volcanic rocks. The data correlate with helium isotopic data, suggesting parallel behaviour of the isotopic systems during silicate differentiation, perhaps as early as 4.39 billion years ago. They conclude that Reunion magmas tap an unusually ancient, primitive source compared to other volcanic hotspots, which could offer a window into the formation and preservation of ancient heterogeneities in Earth's interior. Active volcanic hotspots can tap into domains in Earth’s deep interior that were formed more than two billion years ago1,2. High-precision data on variability in tungsten isotopes have shown that some of these domains resulted from differentiation events that occurred within the first fifty million years of Earth history3,4. However, it has not proved easy to resolve analogous variability in neodymium isotope compositions that would track regions of Earth’s interior whose composition was established by events occurring within roughly the first five hundred million years of Earth history5,6. Here we report 142Nd/144Nd ratios for Reunion Island igneous rocks, some of which are resolvably either higher or lower than the ratios in modern upper-mantle domains. We also find that Reunion 142Nd/144Nd ratios correlate with helium-isotope ratios (3He/4He), suggesting parallel behaviour of these isotopic systems during very early silicate differentiation, perhaps as early as 4.39 billion years ago. The range of 142Nd/144Nd ratios in Reunion basalts is inconsistent with a single-stage differentiation process, and instead requires mixing of a conjugate melt and residue formed in at least one melting event during the Hadean eon, 4.56 billion to 4 billion years ago. Efficient post-Hadean mixing nearly erased the ancient, anomalous 142Nd/144Nd signatures, and produced the relatively homogeneous 143Nd/144Nd composition that is characteristic of Reunion basalts. Our results show that Reunion magmas tap into a particularly ancient, primitive source compared with other volcanic hotspots7,8,9,10, offering insight into the formation and preservation of ancient heterogeneities in Earth’s interior.

Melt origin across a rifted continental margin: A case for subduction-related metasomatic agents in the lithospheric source of alkaline basalt, NW Ross Sea, Antarctica

Abstract: &NA; Alkaline magmatism associated with the West Antarctic rift system in the NW Ross Sea (NWRS) includes a north‐south chain of shield volcano complexes extending 260 km along the coast of Northern Victoria Land (NVL), numerous small volcanic seamounts located on the continental shelf and hundreds more within an ˜35 000 km2 area of the oceanic Adare Basin. New 40Ar/39Ar age dating and geochemistry confirm that the seamounts are of Pliocene‐Pleistocene age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent, as well as to a much broader region of diffuse alkaline volcanism that encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late‐stage breakup of Gondwana at ˜100 Ma, suggesting that the magmatism is interrelated, yet the mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and the roles of lithospheric and sub‐lithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with > 6 wt % MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica‐undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, as well as LREE/HREE (heavy REE) and Nb/Y ratios. Radiogenic isotopes also vary, with Nd and Pb isotopic compositions increasing and Sr isotopic compositions decreasing oceanward. These variations cannot be explained by shallow‐level crustal contamination or by changes in the degree of mantle partial melting, but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica‐undersaturated and incompatible element enriched basalts best represent the composition of the sub‐lithospheric magma source with low 87Sr/86Sr (≤0·7030) and &dgr;18Oolivine (≤5·0‰), and high 143Nd/144Nd (˜0·5130) and 206Pb/204Pb (≥20). The isotopic ‘endmember’ signature of the sub‐lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small‐degree melts (carbonate‐rich silicate liquids) to form amphibole‐rich metasomes. Later melting of the metasomes produced silica‐undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continentward. Ancient and/or more recent (˜550–100 Ma) subduction along the Pan‐Pacific margin of Gondwana supplied the recycled subduction‐related material to the asthenosphere. Melting and carbonate metasomatism were triggered during major episodes of extension beginning in the Late Cretaceous, but alkaline magmatism was very limited in its extent. A significant delay of ˜30 to 20 Myr between extension and magmatism was probably controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle and/or by edge‐driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton.

Calcium Isotopic Compositions of Normal Mid-Ocean Ridge Basalts From the Southern Juan de Fuca Ridge

Abstract: Mantle peridotites show that Ca is isotopically heterogeneous in Earth's mantle, but the mechanism for such heterogeneity remains obscure. To investigate the effect of partial melting on Ca isotopic fractionation and the mechanism for Ca isotopic heterogeneity in the mantle, we report high-precision Ca isotopic compositions of the normal Mid-Ocean Ridge Basalts (N-MORB) from the southern Juan de Fuca Ridge. Ca-44/40 of these N-MORB samples display a small variation ranging from 0.750.05 to 0.860.03 (relative to NIST SRM 915a, a standard reference material produced by the National Institute of Standards and Technology), which are slightly lower than the estimated Upper Mantle value of 1.050.04 parts per thousand and the Bulk Silicate Earth (BSE) value of 0.94 +/- 0.05 parts per thousand. This phenomenon cannot be explained by fractional crystallization, because olivine and orthopyroxene fractional crystallization has limited influence on Ca-44/40 of N-MORB due to their low CaO contents, while plagioclase fractional crystallization cannot lead to light Ca isotopic compositions of the residue magma. Instead, the lower Ca-44/40 of N-MORB samples compared to their mantle source is most likely caused by partial melting. The offset in Ca-44/40 between N-MORB and BSE indicates that at least 0.1-0.2 parts per thousand fractionation would occur during partial melting and light Ca isotopes are preferred to be enriched in magma melt, which is in accordance with the fact that Ca-44/40 of melt-depleted peridotites are higher than fertile peridotites in literature. Therefore, partial melting is an important process that can decrease Ca-44/40 in basalts and induce Ca isotopic heterogeneity in Earth's mantle.

Co-development of Jurassic I-type and A-type granites in southern Hunan, South China: Dual control by plate subduction and intraplate mantle upwelling

Abstract: Two types of spatially and temporally associated Jurassic granitic rocks, I-type and A-type, occur as pluton pairs in several locations in southern Hunan Province, South China. This paper aims to investigate the genetic relationships and tectonic mechanisms of the co-development of distinct granitic rocks through petrological, geochemical and geochronological studies. Zircon LA-ICPMS dating results yielded concordant U–Pb ages ranging from 180 to 148 Ma for the Baoshan and Tongshanling I-type granodiorites, and from 180 to 158 Ma for the counterpart Huangshaping and Tuling A-type granites. Petrologically, the I-type granodiorites consist of mafic minerals such as hornblende whereas the A-type granites are dominated by felsic minerals (e.g., quartz, K-feldspar and plagioclase). Major and trace element analyses indicate that the I-type granodiorites have relatively low SiO2 (64.5–71.0%) and relatively high TiO2 (0.28–0.51%), Al2O3 (13.8–15.5%), total FeO (2.3–4.7%), MgO (1.3–2.6%) and P2O5 (0.10–0.23%) contents, and the A-type granites are characterized by high concentrations of Rb (212–1499 ppm), Th (18.3–52.6 ppm), U (11.8–33.6 ppm), Ga (20.0–36.6 ppm), Y (27.1–134.0 ppm) and HREE (20.3–70.0 ppm), with pronounced negative Eu anomalies (Eu/Eu* = 0.01–0.15). Moreover, the I-type granodiorites are classified as collision-related granites emplaced under a compressional environment, whereas the A-type granites are within-plate granites generated in an extensional setting. Zircon Hf isotopic compositions vary substantially for these granitic rocks. The I-type granodiorites are characterized by relatively young Hf model ages (TDM1 = 1065–1302 Ma, TDMC =1589–2061 Ma) and moderately negative eHf(t) values (–5.9 to –11.5), whereas the A-type granites have very old model ages (TDM1 = 1454–2215 Ma, TDMC = 2211–2974 Ma) and pronounced negative eHf(t) values (–15.8 to –28.3). These petrochemical and isotopic characteristics indicate that the I-type granodiorites may have been derived from a deep source involving mantle-derived juvenile (basaltic) and crustal (pelitic) components, whereas the A-type granites may have been sourced from melting of meta-greywacke in the crust. This study proposes that the pressure and temperature differences in the source regions caused by combined effects of intra-plate mantle upwelling and plate subduction are the major controlling factors of the co-development of the two different types of magmas. Crustal anatexis related to lithospheric delamination and upwelling of hot asthenosphere under a high pressure and temperature environment led to the formation of the I-type magmas. On the other hand, the A-type magmas were formed from melting of the shallower part of the crust, where extensional stress was dominant and mantle-crust interaction was relatively weak. Rifts and faults caused by mantle upwelling developed from surface to depth and successively became channels for the ascending I- and A-type magmas, resulting in the emplacement of magmas in adjacent areas from sources at different depths.