Partial Melting Of Mantle Research Articles

Paleo-Tethys subduction and arc-continent collision: Evidence from zircon U-Pb chronology, geochemistry and Sr-Nd-Hf isotopes of eclogites in western Yunnan, bangbing area, southeastern Tibetan Plateau

The Changning-Menglian suture zone (CMSZ) in the southeastern Tibetan Plateau is a newly discovered HP-UHP metamorphic zone. The eclogites therein are the key evidence constraining the main suture of the Proto- and Paleo-Tethys Ocean in western Yunnan. Targeting the weakly studied Bangbing eclogites, we developed a comprehensive study on the whole-rock compositions, Sr-Nd isotope and zircon U-Pb ages, zircon trace elements and Lu-Hf isotope to reveal the subduction and arc-land collision. The eclogites occur as massive blocks or lenses and embedded in garnet phengite quartz schists of Lancang Group, Early Paleozoic accretionary complex. Their geochemistry is similar to E-MORB, and exhibit isotopic εNd(t) values of 3.14–4.49 and εHf(t) of 14.64–16.41, respectively. The Nb-enriched mafic protoliths suggested they were probably generated by partial melting of the enriched oceanic mantle within the spinel stability field and emplaced or erupted as mid-ocean ridge in the Paleo-Tethys Ocean. By LA-ICP-MS zircon U-Pb age testing, the magmatic zircon grains separated from the eclogites yield a wide range of ages, which may be capture zircon ages rather than protolith crystallization. We infer the age of eclogite-facies metamorphism to be 238 ± 2 Ma based on CL images, zircon trace element analysis, and that this metamorphism marks the collision between the Eastern Lincang magmatic arc, the Simao block and the Western Baoshan block. Thus, exhumation of the eclogites occurred only 7 to 23 Ma later, according to age 231–215 Ma for post-collisional volcanic and granitic rocks east of the CMSZ. Conclusively, the continued subduction of the Paleo-Tethys oceanic crust occurred during the Early-Middle Triassic, and rapid exhumation in the Late Triassic. The Changning-Menglian suture zone is a typical oceanic subduction-accretionary orogeny belt.

Unradiogenic lead isotopic signatures of the source mantle beneath the southernmost segment of the Central Indian Ridge

Numerous studies have shown that the mid-ocean-ridge basalts (MORBs) distributed in the southern Central Indian Ridge (CIR) exhibit the DUPAL anomaly; yet, the corresponding temporal variations of this signal remain underexplored in this region. Here, we report new Sr–Nd–Pb isotope data for the MORB which forms the southernmost segment of the CIR (CIR-1) and interpret these together with previously reported elemental data. Our results indicate that the MORB from the current spreading axis of the CIR-1 segment is produced by a mixture of the partial melt of the depleted MORB mantle and melt derived from the lower continental crust (LCC). This interpretation is consistent with the previously proposed model for Indian MORBs that exhibit the DUPAL anomaly. In contrast, the MORB from the off-axis area shows a depleted trace element composition, particularly for highly incompatible elements. This can be explained by the mixture of the partial melt of a highly trace element-depleted mantle that has not been previously defined in this area and melt from the LCC and C component. Furthermore, high 87 Sr/ 86 Sr, Δ8/4, and Δ7/4 values indicate that the source mantle of the MORB in the off-axis area of the CIR-1 segment has undergone at least two melting events, including the formation of the current MORB. The first melting event occurred approximately 500 Ma and resulted in lower parent/daughter ratios (Th/Pb and U/Pb) in the residual mantle, yielding an unradiogenic lead isotope composition. Overall, our results suggest that a highly depleted mantle domain that underwent an ancient melting event lies under the northern CIR-1 segment, contributing to the formation of MORBs with an unradiogenic lead isotopic signature. • We sampled on- and off-axis mid-ocean-ridge basalt (MORB) from CIR-1. • Depleted mantle partial melt and LCC melt mix to form the on-axis MORB. • In contrast, the off-axis MORB is derived from trace element-depleted mantle. • Trace element-depleted melt could be mixed with melts of the LCC and C component. • A highly depleted mantle domain that underwent ancient melting is suggested.

Paleoproterozoic TTG gneisses and granites of the Huai’an Complex: Geochemical and zircon U-Pb-Hf data provide insights into subduction history and crustal growth

Tonalite-trondhjemite-granodiorite (TTG) suites and granitoids are major components of the Precambrian continental crust. Here we investigate the Paleoproterozoic TTG gneisses and granites from the Huai’an Complex based on new and published geochemical and geochronological data with a view to trace the tectonic evolution of the North China Craton (NCC). The TTG gneisses show high Mg#, Sr and Sr/Y, low Yb, positive Eu anomalies, as well as enriched light REE (LREE) and depleted heavy REE (HREE) patterns. The magmatic zircons of TTG gneisses show εHf(t) of 9.1 to −2.8 and 176Lu/177Hf of 0.046. Geochemical characteristics suggest that the parental magmas of TTG gneisses were sourced from the partial melting of subducted hydrated oceanic crust and mantle wedge, with fractional crystallization of plagioclase and amphibole. The granites are magnesian, medium-K to high-K calc-alkaline, metaluminous to weakly peraluminous, and have decreasing P2O5, Al2O3, TiO2, MgO, CaO, and Sr with increasing SiO2, increasing Pb with increasing SiO2, decreasing Ta/Nb with increasing TiO2, positive Eu anomalies and low HREE. The magmatic zircons of granites show εHf(t) of 2.72 to −2.6 and 176Lu/177Hf of 0.033. Geochemical characteristics classify the granites into I-type granites and indicate that magma derived through partial melting of infra-crustal igneous rocks and subcontinental lithospheric mantle with fractional crystallization of apatite, K-feldspar, plagioclase and mica. Geochemical characteristics and zircon U-Pb data indicate that the Huai’an Complex was a long-lived magmatic arc and in crustal growth stage during 2.51–2.39 Ga. The crystallization ages of TTG gneisses and granites in the Huai’an Complex show a general younging-trend from north to south along NE-SW direction, likely suggesting a scissor-like subduction model with an oceanic lithosphere subduction earlier beneath the northern part of the Huai’an Complex than the southern part.

Two-stage Cu[sbnd]Pb[sbnd]Zn mineralization of the Baoshan deposit in southern Hunan, South China: Constraints from zircon and pyrite geochronology and geochemistry

The Baoshan CuPbZn deposit is one of the representative polymetallic deposits in the middle of the Nanling Range, the western part of the Qin–Hang ore belt. In order to better understand the magmatic activity (granodiorite porphyry and porphyritic granodiorite) and mineralization in the Baoshan deposit, in situ UPb and LuHf isotope studies of zircons and trace element compositions of two generations of pyrite (ore stage: Py I and post-ore stage: Py II) have been carried out. This constrains the geochronology, magma source, fluid composition, and the origin of the ore-forming materials. The zircon UPb dating results from two types of intrusions show two geochronological populations: (i) ~180 Ma for the Cu (PbZn) mineralization that is related to the porphyritic granodiorite, and (ii) ~160 Ma for the CuPbZn mineralization which is associated with the granodiorite porphyry. The similar REE patterns, Hf/U, Nb/Ta, and Y/Ho ratios along with Hf isotopic characteristics of both granite types indicate that these rocks were sourced from the partial melting of lithospheric mantle and lower crust. Zircons from the granodiorite porphyry have higher Nb, Ta, Lu, and Hf contents, but lower Ti contents and Th/U ratios than those from the porphyritic granodiorite, suggesting that the magma-forming granodiorite porphyry had lower temperature, higher oxygen fugacity, and probably experienced strong crystal fractionation. Py I and Py II are associated with the porphyritic granodiorite and are related to the mineralization that occurred at ~180 Ma. The composition of pyrite in the two stages indicates the contribution of granite and limestone to mineralization. The higher contents of metals (Cu, Pb, Zn, Bi, Ag, except Co and Ni) in Py I than in Py II indicate the cooling and mixing process of ore-forming fluid with meteoric water. The geochemical characteristics of the pyrite generations and UPb dating of zircons suggest that the small-scale mineralization occurred at ~180 Ma under a relatively high-temperature and lower oxidized magmatic environment, whereas the major metallogenic event occurred at ~160 Ma under a low-temperature, medium oxidized, and strongly fractionated magmatic environment. This study reveals that the multi-stage magmatic and mineralization processes mainly occurred during the Early to Late Jurassic in South China.

Thermodynamic modeling of melt addition to peridotite: Implications for the refertilization of the non-cratonic continental mantle lithosphere

In a classic model of evolution of the non-cratonic continental mantle lithosphere, harzburgites represent the refractory (<5% clinopyroxene) residues of high degrees of partial melting of fertile mantle, while lherzolites (>5% clinopyroxene) represent residues of lesser degrees of partial melting. However, partial melting is not the only process that could explain the peridotite compositional variability that ranges from fertile (>2 wt% Al2O3, <45 wt% MgO) to refractory (<2 wt% Al2O3, >45 wt% MgO). In the refertilization process, harzburgite is a refractory protolith (potentially previously formed by partial melting of a fertile mantle) that undergoes reactive percolation of silicate melts derived from the underlying asthenosphere, resulting in the crystallization of a new generation of minerals (mostly clinopyroxene). A simple but critical first step towards understanding the refertilization process is to examine how modal and major element compositions evolve as melts are added to peridotites. Here we use a thermodynamically-constrained two-component mixing model to independently evaluate the roles of five different parameters: pressure, temperature, redox conditions, and compositions of the initial peridotite and the added basaltic melt (hereafter referred to P-T-fO2-Xπ-Xmelt), during melt addition. We compare the results with observed suites of peridotites. The main observations are as follows: (1) the produced model is consistent with the global peridotite database, and (2) T, fO2 and small variations of pressure have almost no impact on the evolution of the system. In contrast, the mineralogy of the percolated harzburgite has a substantial effect on the variation of the modal proportions. The parameter with the most significant impact is Xmelt, which is directly linked to the geodynamic context and melting conditions. This parameter directly controls the refertilization reaction and so, the phase proportions and the bulk-rock composition. Elements that partition preferentially in the melt phase (e.g., Na) display depletions in natural assemblages that are stronger than those predicted from the simple mixing model, consistent with the fact that the natural process occurs in an open system, and that reactive percolation likely results in incompatible element enrichment in the associated melt. Our results corroborate the suggestion that most of the spectrum of compositional variability observed in lithospheric mantle peridotites can be explained by the impregnation of primitive silicate melt in refractory harzburgites.

Petrogenesis of the Yeonhwa ultrapotassic intrusions in the Yeongnam Massif—Evidence for enrichment of the Triassic continental lithospheric mantle beneath the Korean peninsula

The Triassic period is when abundant late-orogenic magmatism occurred in the Korean peninsula, but the characteristics of the lithospheric mantle at that time beneath the Yeongnam Massif have not been well-defined because of the scarcity of mafic intrusions. We report zircon U Pb and phlogopite 40 Ar/ 39 Ar ages, petrography, and geochemistry of the Triassic (227.6 Ma) ultrapotassic igneous rocks in the Yeonhwa I Mine (Yeonhwa Ultrapotassic Intrusions, YUI) emplaced into the Taebaeksan Basin in the Yeongnam Massif. The YUI occur as two stocks with different lithologies: phlogopite-rich and amphibole-rich, where the former underwent carbonate-melt interaction along the margin in contact with the carbonate formation. The two lithologies share similar compositional and isotopic characteristics; however, the phlogopite-rich YUI better retains its mantle-derived features, having idiomorphic high-Mg# (up to 91.9) phlogopite and diopside phenocrysts and bulk concentrations of high MgO (Mg# = molar 100 Mg/[Mg + Fe] > 70), Cr (208–1150 ppm), and Ni (48–219 ppm). The enrichment of the YUI in H 2 O (phlogopite-rich), silica contents (50.2–56.9 wt%), and large ion lithophile elements (LILE; K, Rb, Sr, Ba, Pb), together with depletion of high fields strength elements (HFSE; Ti, Zr, Nb, Hf, Ta), are typical of the ultrapotassic rocks in orogenic settings. The elemental and isotopic data imply that the YUI originated from partial melts of the phlogopite-bearing lithospheric mantle near the spinel-garnet transition zone. The extremely enriched isotopic ratios ( 87 Sr/ 86 Sr(t) = 0.70756 to 0.71227, ε Nd (t) = −8.82 to −11.0) suggest a highly enriched mantle source, possibly more enriched than the contemporary pelagic sediments. Considering that the Yeongnam Massif was in a subduction system during Permian-Triassic, not a continental collisional belt, the enrichment was inherited from a combination of both young, subduction-related and ancient, pre-existing metasomes in the continental lithospheric mantle. Our data confirm the presence of the enriched, volatile-rich lithospheric mantle beneath the Triassic Yeongnam Massif. This enriched origin contrasts with earlier ( c . 240–280 Ma) Yeongnam arc plutons that originated from the depleted asthenospheric mantle (ε Nd (t) ≥ +2). Since the enrichment is also characteristic of most of the c . 220–240 Ma igneous rocks around the Korean peninsula, such lithospheric mantle lithology was widespread during the Late Triassic. • The Yeonhwa Ultrapotassic Intrusions (YUI) intruded into the Yeongnam Massif at around 227.6 Ma. • Reaction with country rocks developed metasomatized margins of the intrusion. • The YUI were derived from the phlogopite-bearing sub-continental lithospheric mantle. • The Triassic Yeongnam Massif had the old lithospheric mantle further modified by the Permina-Triassic subduction zone.

Petrogenesis of late Permian to Middle Triassic magmatic rocks on northern Hainan Island, South China: Implications for crust–mantle interaction and the tectonic evolution of the Paleo-Tethys

Geochronological, geochemical, and Sr–Nd–Hf isotope data are reported for the late Permian to Middle Triassic Juding gabbros, the Xinxing gabbroic–dioritic rocks, and granites and mafic enclaves from the northern part of Hainan Island, South China. Our data were used to constrain the petrogenesis of these rocks, the crust–mantle interactions involved, and the implications for the tectonic evolution of the area. Zircon U–Pb dating gave crystallization ages of ca. 235 Ma for the Juding gabbros, ca. 252 Ma for the Xinxing gabbroic–dioritic rocks, and 253–238 Ma for the granites and mafic enclaves. The Juding gabbros are characterized by weak negative Nb, Ta, and Ti anomalies, low initial 87Sr/86Sr ratios (0.704603–0.704630), and high εNd(t) values (+3.7 to +3.8). The Juding gabbros are inferred to have been derived by partial melting of depleted asthenospheric mantle, with these melts mixing with melts produced from enriched lithospheric mantle. The Xinxing gabbroic–dioritic rocks have relatively intermediate compositions, with initial 87Sr/86Sr ratios of 0.706600–0.708664, εNd(t) values of −6.3 to −5.1, and εHf(t) values of −3.3 to −0.6. The Xinxing gabbroic-dioritic rocks probably derived by partial melting of enriched lithospheric mantle that was modified by subduction-related melts and/or fluids. The granites from the northern part of Hainan Island have extremely high SiO2, low MgO and Ni contents, with initial 87Sr/86Sr ratios of 0.704794–0.711158, εNd(t) values of −8.5 to −6.6, and εHf(t) values of −15.0 to −1.2. They were probably derived by partial melting of a variety of ancient crustal sources with some addition of mantle material. The mafic enclaves in these granites have relatively low SiO2, high MgO and Ni contents, with initial 87Sr/86Sr ratio of 0.707882, εNd(t) value of −6.4, and εHf(t) values of −9.8 to −3.9. They seem to represent the remnants of a mafic component derived from enriched mantle. The gabbroic–dioritic rocks and gabbros formed in response to the subduction and break off of the Paleo-Tethyan oceanic slab. The late Permian to Middle Triassic magmatic rocks from the northern part of Hainan Island record complex multistage crust–mantle interactions and lithosphere–asthenosphere interaction, probably related to subduction during closure of the Paleo-Tethys Ocean and the consequential collision of the South China and Indochina blocks.

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO2 contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO2 contents and high Mg# (Mg# = 100 Mg/(Mg + Fe2+)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (87Sr/86Sr)i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (87Sr/86Sr)i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO2 (77.63–82.52 wt.%) and low total Fe2O3, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.

Carbonatitic footprints in the Bayan Obo REEs deposit as seen from pyrite geochemistry

The genesis of the Bayan Obo REEs deposit has been debated for a century. The most outstanding issue concerns the genesis of the ore-hosting dolomite and the contribution of the Paleozoic overprint events in the rare earth budget. Pyrite offers a unique window into those debates in that it is the only common fingerprinting mineral shared by both events. In this contribution, LA-ICP-MS trace element mapping, in-situ elemental and sulfur isotopic analyses on pyrite are integrated in order to constrain the genesis of ore-hosting dolomite and evaluate the contribution of the Paleozoic thermal events to enormous REEs resources. Five types of pyrite were distinguished in ore-hosting dolomites, various types of ore and skarns based on mineral paragenesis and geochemical signatures. The dolomites contain early euhedral pyrite (Py-1) overgrown by magnetite and euhedral pyrite (Py-2w in the West Open Pit and Py-2d in the deep boreholes) associated with pyrrhotite, monazite, dolomite and apatite. By contrast, pyrite in the banded ores (Py-3b) and the massive ores (Py-3 m) was co-precipitated with magnetite, aegirine, fluorite, riebeckite, REEs minerals and apatite. Pyrite in the vein ores (Py-4) usually coexists with barite, quartz, aegirine and calcite, and those (Py-5) in skarns associated with biotite and amphibole. Py-1 is enriched in Ti, Ni, Nb, Hf and REEs, with high Co/Ni and Y/Ho ratios (ca. 66), and high δ34S values from +8.6‰ to +13.8‰. In comparison, Py-2w and Py-2d have higher Co/Ni yet lower Y/Ho (ca. 11) ratios, but exhibit magmatic δ34S values (-1.0‰ to +3.0‰; −1.0‰ to +3.7‰). Py-3b and Py-3 m are relatively enriched in Cr, Mn, Zn and LREEs, and possess intermediate δ34S values (+5.7‰ to +7.7‰, and +0.2‰ to +2.4‰, respectively). Py-4 is generally depleted in trace elements with magmatic δ34S values (-2.0‰ to +2.7‰). Compared with Py-3 m and Py-3b, Py-5 is characterized by high Co yet low Ti, Cr, Mn, Cu and Zn with variable REEs, and is enriched in 34S (δ34S: +9.1‰ to +10.8‰). The high δ34S value and Y/Ho ratio in Py-1 indicate that the ore-hosting dolomite is an igneous carbonatite originated from partial melting of upper mantle contaminated by altered and subducted oceanic crust. The marked difference in trace elements and sulfur isotopes between Py-3, Py-4 and Py-5 implies that the Paleozoic thermal events contributed little to the primary REEs mineralization except remobilizing preexisting ore metals.

Late cretaceous magmatism in the Northern Lhasa subterrane, central Tibet: geodynamic implications

ABSTRACT Compositional changes of successively emplaced magmatites can be used to infer changes in their petrogenetic evolution and geodynamic setting. The central-northern Lhasa terrane hosts extensive Late Cretaceous magmatism, however, the mechanism that triggered the magmatism and its relationship to the directly preceding Late Cretaceous tectonic transition in the central Tibetan Plateau is still unresolved. Therefore, we conducted a detailed petrological, geochronological, geochemical, and isotopic study on recently identified Late Cretaceous doleritic and granodioritic rocks in the Xiongmei area, northern Lhasa subterrane. The potassic (K2O/Na2O ratios >1) dolerites (ca. 80 Ma) display a post-collisional basaltic shoshonitic signature with uniform moderately enriched whole-rock Sr–Nd and zircon–Hf isotopic compositions ((87Sr/86Sr)t = 0.7070 to 0.7095, whole-rock εNd(t) = – 2.4 to – 1.3, and zircon εHf(t) = – 5.9 to +1.9), suggesting that they evolved from a similar parental magma and were likely derived by partial melting of an enriched lithospheric mantle metasomatized by hydrous fluids. The granodiorites are geochemically divided into medium-K and high-K calc-alkaline rocks. The medium-K samples (ca. 80 Ma) are generally characterized by low K2O/Na2O ratios <1, whereas the high-K samples (78–76 Ma) display high K2O/Na2O ratios >1. The medium-K samples have positive zircon εHf(t) values from +4.8 to +6.5 and uniform whole-rock ɛNd(t) values from – 2.4 to – 1.3, which significantly differ from those of the high-K samples that have negative to positive zircon εHf(t) values from – 1.9 to +2.8 and negative whole-rock ɛNd(t) values from – 8.4 to – 5.1. The medium-K samples are characterized by high Sr contents and Sr/Y ratios, coupled with low Y and HREE contents, similar to adakitic compositions, and were interpreted to have been derived from deep partial melting of the foundering juvenile garnet-bearing mafic lower crust at relatively low temperatures. Compared to the medium-K samples, the high-K samples have lower Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and distinct negative Eu, Sr and Ti anomalies and are most likely derived from shallower partial melting of the K-rich juvenile lower crust at relatively high temperatures. Considering the occurrence of variable magmatism in the central-northern Lhasa subterrane during the Late Cretaceous, the change in magma composition and lower crustal melting conditions (cold-deep vs. hot-shallow) suggest that the lithospheric mantle keel was partially detached by small-scale mantle convection in response to the low-angle Neo-Tethys-associated subduction between 85 and 70 Ma.

From rifting to oceanization in the Gulf of Aden: Insights from 2D numerical models

We investigate the evolution of the Gulf of Aden from rift initiation to the development of active oceanic spreading center by means of 2D thermo-mechanical numerical models, in which the formation of oceanic crust and serpentinite due to the hydration of the uprising mantle peridotite has been implemented. Our analysis highlights that evolution of the models is characterized by four main tectonic phases: a) a first phase (phase I) characterized by low deformation rates throughout the divergent crustal blocks, except near the future ridge where a high crustal velocity gradient generates an intense strain rates; b) a second phase (phase II) during which the crust undergoes an intense, stable and widespread strain, with the localization of the thinning near the future ridge that ends into crustal breakup c) a third phase (phase III) that characterizes the post- crustal breakup evolution of the models during which a mechanical relaxation of the system and a continuum decreasing of the strain rate can be observed, until the occurrence of lithospheric breakup, and d) fourth phase (phase IV) that lasts up to the end of the evolution and during which the two continental blocks move rigidly. We also find that the timing of mantle serpentinization is not affected by the initial thermal configuration of the lithosphere, but a relationship with the crustal thickness can be observed. Rather, the timing of mantle partial melting strongly depends on the initial thermal conditions of both the lithosphere and the crust. We constrain the crustal and lithospheric thickness at 40 and 150 km, respectively, considering the timing of breakup that occurs 20 Myr after the onset of the extension for 0.05% percentage of mantle hydration (in agreement with magma-poor rift margins). Finally, model prediction supports the hypothesis that the Gulf of Aden developed as a slow passive rift of a thin lithosphere with a thick crust and the variation of the features along the passive margins could be related to a lateral variation in the amount of H2O in the mantle, which determines different timing in the mantle melting.

Petrogenesis of early cretaceous intermediate to felsic rocks in Shanghai, South China: Magmatic response to Paleo-Pacific plate subduction

Voluminous magmatism and extensional deformation are the prominent late Mesozoic tectonic features in East Asia, which recorded the evolutionary history of the continental marginal lithosphere. Due to the thick covering of Quaternary and Pliocene sediments in the region, the Early Cretaceous magmatism in northeastern South China is not well constrained. In this study, we collected samples from the boreholes in Shanghai, East China and provided geochronological, elemental and isotopic analyses of andesite and granodiorite. Zircon UPb age results indicate that andesites and granodiorites were formed at ca. 135 to 102 Ma. The andesites exhibit moderate MgO contents, slightly radiogenic Sr (87Sr/86Sr(t) = 0.706149 to 0.706170) and variable unradiogenic Nd and Hf isotopic compositions (εNd(t) = −5.32 to −5.12; εHf(t) = −10.8 to −0.6). The granodiorites exhibit arc-like trace-element patterns with high Sr/Y ratios, as well as slightly radiogenic initial Sr (87Sr/86Sr(t) = 0.706754 to 0.706790) and moderately unradiogenic Nd and Hf isotopic compositions (εNd(t) = −6.54 to −6.27; εHf(t) = − 4.0 to −6.7). Both andesites and granodiorites exhibit similar initial Pb isotopic compositions (andesites: 206Pb/204Pb = 18.232–18.259, 207Pb/204Pb = 15.547–15.548, and 208Pb/204Pb = 38.338–38.350; granodiorites: 206Pb/204Pb = 17.930–17.946, 207Pb/204Pb = 15.542–15.543, and 208Pb/204Pb (38.256–38.293). The results recommend that the ca.130 Ma andesites were produced by the partial melting of an enriched lithospheric mantle, which was previously metasomatized with sediments in the source region in a continental arc setting of the Paleo-Pacific subduction. The ca.102 Ma adakitic granodiorites probably originated from partial melting of the lower crust with the addition of mantle-derived magmas. With the consolidation of previous studies, we argue that the flat slab subduction of the paleo-Pacific Plate led to the formation of these intermediate to felsic magmatism.

Albian–Cenomanian granitoid magmatism in Eastern and Central Tibet as a result of diachronous, continental collision induced slab tear propagation

AbstractA discrete belt of Albian–Cenomanian granitoid plutons occurs in the Lhasa and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau (ETP) and represents a major magmatic pulse in the plateau’s crustal evolution during the Cretaceous. The geochemistry, petrogenesis, and magmatic development of these granitoids are different from those of magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang terranes, indicating different heat and melt sources and tectonic setting of their formation. We present here new mineral, whole-rock and isotope geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP and discuss its geochemical-petrological characteristics and magmatic development in comparison to the other Cretaceous plutons in the ETP and CTP, and within the tectonic framework of the Mesotethyan geology of Tibet. Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous granitoids in the ETP and CTP range in composition from granite, K-feldspar granite to monzogranite and biotite monzogranite, representing highly fractionated I-type granites with relatively high SiO2 and K2O contents, variable (Na2O + K2O)/CaO and FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display significant negative Eu anomalies (Eu/*Eu) = 0.04–0.12) and strong depletions in Sr and Ba, and are strongly enriched in large ion lithophile elements but depleted in high field strength elements. Their εHf(t) values correspond to wide ranging Hf isotope crustal model ages (TDMC) of 0.3–1.9 Ga, and their Sr-Nd isotopic signatures show elevated (87Sr/86Sr)i ratios (0.7034–0.7105) and negative εNd(t) values of –8.8 to –4.9. These high whole-rock (87Sr/86Sr)i ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of melts derived from partial melting of subducted sediments into the melt evolution of these granitoids that involved partial melting of the subduction-metasomatized lithospheric mantle and the mafic- to intermediate-composition continental crust. The extant zircon crystallization ages of the granitoid intrusions in the CTP and ETP show eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an apparent eastward migration of the heat source through time. A diachronous collision of the NLT with the SQT during 145–120 Ma and the subsequent slab breakoff induced, eastward propagated slab tear and asthenospheric upwelling produced the hybrid melts of the Albian–Cenomanian granitoids and their emplacement in a discrete, narrow magmatic belt in the CTP and ETP.

Crustal growth/reworking and stabilization of the western Superior Province: Insights from a Neoarchean gneiss complex of the Winnipeg River terrane

AbstractLong-term stability of the continental lithosphere is attained through a cumulative increase in net buoyancy and rigidity due to progressive compositional differentiation (i.e., cratonization). As stable cratons provided the nucleus for the subsequent accretionary growth and tectono-magmatic reworking that produced modern continental crust, the geodynamic processes that facilitated the stabilization of cratons are critical for understanding the evolution of Earth’s lithosphere. This study uses a portion of the Winnipeg River terrane, one of the oldest terranes of the western Superior Province, as a natural laboratory to investigate Archean crustal growth (partial melting of mantle) and reworking (partial melting of crust) and provides insights into the geodynamic processes driving mantle depletion and crustal remelting. Zircon U-Pb data obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) from an extensive Winnipeg River terrane gneiss complex reveal six major magmatic events at ca. 3060 Ma, 2930–2920 Ma, ca. 2910 Ma, 2830–2800 Ma, 2735–2730 Ma, and ca. 2700 Ma and regional metamorphism at ca. 2900 Ma. Whole-rock geochemistry and zircon Lu-Hf and trace element data indicate that (1) the magmatism at ca. 3060 Ma and ca. 2930–2920 Ma represents reworking of the isotopically evolved components of the incipient Winnipeg River terrane at shallow depths, (2) the ca. 2910 Ma magmatism features a step-change of Hf isotopic compositions from subchondritic to suprachondritic and records the formation of new juvenile magmas and the first reworking of existing juvenile crust, and (3) the magmatism after ca. 2830 Ma largely reflects reworking of the juvenile components of the incipient Winnipeg River terrane at medium to shallow depths prior to the ca. 2700 Ma trans-crustal magmatism associated with the convergence of the Winnipeg River terrane and western Wabigoon terrane. Juvenile magmatism and crustal growth in the Winnipeg River terrane at ca. 2910 Ma are inferred to correspond with significant mantle depletion below the Winnipeg River terrane, which led to a more stable lithospheric mantle in this part of the western Superior Province. Zircon trace element data support a mantle upwelling model rather than lithosphere recycling models for the depletion of mantle at ca. 2910 Ma. This study suggests that crustal growth and mantle depletion bracketed by prolonged, episodic crustal reworking may be a fundamental characteristic of the cratonization process.

Evidence for transitional and mildly alkalic eruptions during Hawai'i's dominantly tholeiitic shield-building stage: Insights from the Kulanaokuaiki Tephra (≥1.0 ka) at Kīlauea Volcano, HI

Vitric clasts from a marker horizon in the Kulanaokuaiki Tephra, deposited on the summit and flanks of Kīlauea Volcano, HI, during a prolonged period of explosive eruptions and low magma supply >1.0 ka, show unusual enrichments in alkalis relative to silica and in incompatible elements, in contrast with the volcano's dominantly tholeiitic shield-building lavas. The clasts are transitional basalts, with compositions near the tholeiitic-alkalic boundary (Macdonald and Katsura, 1964). Nearly uniform in composition across ∼200 km2, the clasts are the most proximal to the summit among rare occurrences of transitional and mildly alkalic shield-stage eruptions at Kīlauea. In contrast with the volcano's effusive shield-building style, stratigraphic evidence suggests the clasts were deposited in one of Kīlauea's most vigorous explosive eruptions in the past 2.5 ka—an episode punctuated by high fountaining along with an ∼12 km-tall ash plume. The tephra's composition is consistent with those of five unusual shield-stage Kīlauea lavas. All contain <50.0 wt% SiO2 and 3.0–4.0 wt% Na2O + K2O. All but one contain ≥0.70 wt% K2O, ≥3.0 wt% TiO2, and > 0.30 wt% P2O5. The transitional Kulanaokuaiki clasts also display elevated abundances and ratios of incompatible trace elements (e.g., ∼19 ppm La, La/Yb ∼ 8.6). Compositionally, these early shield-stage clasts and lavas resemble those erupted at the end of shield building at older Hawaiian volcanoes, attributed to lower degrees of partial mantle melting. Chemical constraints and extrinsic circumstances suggest that similar episodes of transitional and mildly alkalic volcanism will likely recur throughout shield building.

Petrogenesis of Middle Triassic intermediate-mafic igneous rocks in East Kunlun, Northern Tibet: Implications for the crust growth and Paleo-Tethyan orogeny

Triassic intermediate-mafic igneous rocks are sporadically exposed in the East Kunlun Orogen Belt (EKOB), and their petrogenesis and geological implications for the Paleo-Tethyan orogeny are not well constrained. Geochronologic and geochemical data for the gabbro, gabbroic diorite and quartz diorite in Qingshuihe pluton are presented here to constrain their ages, petrogenesis and geodynamic settings. LA-ICP-MS zircon U-Pb dating indicates that the gabbro, gabbroic diorite and quartz diorite were formed at 239.8 ± 2.1 Ma, 238.4 ± 1.1 Ma and 235.7 ± 1.1 Ma, respectively. The studied samples span a wide range in geochemical compositions with SiO2 content of 47.55–63.93 wt%, exhibiting low-K tholeiitic to high-K calc-alkaline affinities. All samples have trace element compositions resembling typical arc magmas, however, from gabbro, gabbroic diorite to quartz diorite, the fractionation degree of rare earth elements gradually increases ((La/Yb) N = 2.08–2.52, 2.20–9.23 and 7.15–17.95, respectively), and the enrichment in large ion lithophile elements (e.g., Rb, Ba, Th and K) and the depletion in high field strength elements (e.g., Nb, Ta, and Ti) are also more obvious. These geochemical features, combined with their enriched Sr-Nd-Hf isotopes ((87Sr/86Sr)i = 0.709694–0.71445, εNd(t) = -7.9 – -4.7, εHf(t) = -3.65–3.79, respectively), unanimously indicate their parental magma was derived by partial melting of an enriched lithospheric mantle, followed by cumulation of tabular plagioclase and prismatic amphibole generating the appinitic gabbro, and subsequent certain degree of magma mixing and fractional crystallization forming the evolved gabbroic diorite and quartz diorite. This study therefore not only demonstrates that mantle-derived arc magma underplating followed by crust-mantle mixing and fractional crystallization is an important mechanism of continental crust growth, but also confirms that the East Kunlun Paleo-Tethyan oceanic subduction lasted at least until 235 Ma.

Melt generation and trace element fractionation of intermediate arc magma from Andaman subduction zone

The submarine volcanoes, located in the southern part of Andaman Sea, north eastern Indian Ocean, result from the subduction of the Indo-Australian Plate beneath the Southeast Asian Plate and represent one of the less studied submarine volcanism among the global arc systems. The present study provides new petrological and geochemical data for the recovered rocks from the submarine volcanoes and documents the petrogenetic evolution of Andaman arc system. Geochemical attributes classify the studied samples as basaltic andesite, andesite, dacite to rhyodacite reflecting sub-alkaline, intermediate to acidic composition of the magma. Petrographic studies of the basaltic andesites and andesites show plagioclase [An38-An57 in basaltic andesites; An27-An28 in andesites] and clinopyroxene as dominant phenocrystal phase in a cryptocrystalline groundmass. Plagioclase (An25-An45) marks the principal phenocrystal phase in dacite with sub-ordinate proportion of biotite and amphibole of both primary and secondary origin along with minor amount of K-feldspar. The submarine volcanic rocks from Andaman arc system exhibit pronounced LILE, LREE enrichments and HFSE (negative Nb, Ta and Ti anomalies), MREE and HREE depletion thereby endorsing the influence of subduction zone processes in their genesis. Elevated abundances of Th with relatively higher LREE/HFSE than LILE/HFSE, LILE/LREE suggest significant contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids towards mantle wedge metasomatism thereby modifying the sub-arc mantle. Partial melting curves calculated using the non-modal batch melting equation suggest primary magma generated due to ~31–35 % degree of partial melting of spinel lherzolite mantle beneath the arc system. Fractional crystallization model suggests fractionation of 45 % plagioclase, 40 % clinopyroxene, 5–10 % amphibole and 5–10 % biotite which is consistent with the petrographic observations. Further, the assimilation-fractional-crystallization (AFC) model for the studied rocks indicates nominal crustal contamination. Therefore, this study infers that the melt evolution history for the Andaman arc volcanic rocks can be translated in terms of (i) generation of precursor magma by ~31–35 % partial melting of a spinel lherzolite mantle wedge, metasomatized predominantly by subducted slab sediments and (ii) the parent magma generation was ensued by fractionation dominated melt differentiation with nominal input from arc crust.

Partial Melting of Lithospheric Mantle and Formation of the Early Cretaceous Alkaline Rocks in the Guandimiao REE Deposit, Luxi Terrane, Eastern China

The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO2, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe2O3 (Total), MgO, P2O5, TiO2, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.

Geochronological, Geochemical and Sr-Nd-Pb-Hf Isotopic Constraints on the Petrogenesis of Pyroxene Diorites in the Sanchahe Iron-Gold Deposit, Western Shandong

The Sanchahe iron-gold deposit in western Shandong province is located in the southeast of the North China Craton. The skarn type of iron-gold deposit (without copper) is located in the contact belt between pyroxene diorites and Ordovician limestone xenoliths. The zircon U-Pb age of pyroxene diorites is 138.4 ± 1.2 Ma belonging to the Early Cretaceous. Pyroxene diorites (Rittmann index σ = 2.2~2.5) belong to the calc-alkaline series and the SiO2 content is 55~59%. Furthermore, pyroxene diorites are characterized by high Mg and Na contents and high contents of Sc, Cr, Co, and Ni, indicating that it has a geochemical affinity with mantle-derived magma. All of the samples are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), with a weak Eu-negative anomaly. They are also enriched in Cs, Ba and Sr and depleted in high field strength elements (HFSEs) (Nb, Ta, Zr), indicating the involvement of some crustal materials. The whole-rock (87Sr/86Sr)I values are 0.70824~0.70891, and the εNd(t) values are from −8.1 to −12.5. Two-stage model ages of TDM2 are 1591~1928 Ma. The εHf(t) values range from −7.0 to −17.4, and the two-stage model ages of TDM2 range from 1629 to 2297 Ma, indicating that the magma was derived from Mesoproterozoic to Neoarchean crusts. Combined with Pb isotopic characteristics, pyroxene diorites may be generated by the partial melting of the enriched lithospheric mantle, which had been metasomatized by the ancient lower crust materials of the North China Craton and formed in the strong extensional tectonic setting linked with lithospheric thinning, which may be caused by the rollback of the Pacific plate after subduction.

Modification of the lithospheric mantle induced by recycled crustal components: Insights from Early Cretaceous appinites from the Liaodong Peninsula, NE China

AbstractThe term “appinites” refers to amphibolerich mafic rocks, as one of the most typical products of partial melting of a metasomatic lithospheric mantle. This paper presents results of integrated geochemical and isotopic analyses for six Early Cretaceous (125–119 Ma) appinitic plutons from the Liaodong Peninsula, northeastern North China Craton (NCC), to not only investigate the properties of the metasomatic lithospheric mantle induced by variable recycled crustal components but also to discuss a hydrous mantle source. The appinites originated from partial melting of sub-continental lithospheric mantle (SCLM), and can be geochemically categorized into three groups distinguished by the input of different recycled crustal components (e.g., terrigenous sediments and delaminated lower continental crust) on the basis of two distinct Hf-O isotopic arrays. Based on the occurrence of voluminous amphiboles (50–60 vol%) and minor clinopyroxene remnants, a petrogenetic model was proposed to interpret the formation of appinites. Asthenospheric upwelling caused by the subduction and roll-back of the Paleo-Pacific plate firstly resulted in partial melting of the SCLM to yield basaltic magmas. Subsequently, numerous hydration reactions between early anhydrous minerals (e.g., olivine and pyroxene) of basaltic magmas and hydrous melts derived from terrigenous sediments and delaminated lower continental crust occurred to produce the studied appinites. Ultimately, the recycled crustal materials can serve as the metasomatic agent not only to transfer the enriched isotopic signatures into the studied appinites but also to represent hydrous components to play the part of hydrous reactions. The high abundance of amphibole in the studied appinites is indicative of a hydrous lithospheric mantle beneath the eastern NCC.