Partial Melting Of Lithospheric Mantle Research Articles

Whole-Rock and Apatite Geochemistry of Late Triassic Plutonic Rocks in the Eastern Songpan-Ganzi Orogenic Belt: Petrogenesis and Implications for Tectonic Evolution

Abstract To constrain the late Triassic tectonic evolution of the Songpan-Ganzi orogenic belt, we present new whole-rock and in situ apatite geochemistry for plutonic rocks in its eastern margin. The Taiyanghe pluton can be classified into two rock types: dioritic and granitic rocks. The former exhibits low SiO2 and MgO contents but high Al2O3, Th, LREE contents, and Th/Yb and Th/Nb ratios, as well as low Ba/La and Ba/Th ratios and enriched Sr-Nd isotopic compositions, which, together with apatite geochemistry and Nd isotopes, indicate that they were derived from low degrees of partial melting of lithospheric mantle metasomatized by sediment-derived melts. The latter is characterized by high Sr and low Y and Yb, with elevated Sr/Y and (La/Yb)N ratios, implying an adakitic affinity. Notably, their similar Sr-Nd isotopic compositions indicate an origin from partial melts of a newly underplated lower crust. The Maoergai granitic rocks, characterized by high Sr and low Y and Yb contents with high Sr/Y and (La/Yb)N ratios, are indicative of adakitic rocks. In combination with the enriched whole-rock Sr-Nd isotopes and the apatite Nd isotopic data, we suggest that they were generated by the partial melting of the ancient thickened mafic lower crust. The Markam and Yanggonghai felsic granitoid rocks are peraluminous and similar to typical S-type granitoids, indicating an origin from remelting of the Triassic metasedimentary rocks. Based on the temporal-spatial relationship of the late Triassic plutonic rocks in the orogenic belt, we suggest that these rocks were formed in association with the roll-back and subsequent break-off of a subducted slab of the Paleo-Tethys Ocean. During the subduction, the formation of the Maoergai adakitic rocks was triggered by slab roll-back, whereas the magmatic “flare up” (ca. 216–200 Ma) was likely caused by slab break-off. This indicates that the final closure of the Paleo-Tethys Ocean happened in the end of the Triassic or Early Jurassic.

Neoproterozoic slab window in the western Yangtze Block, South China: Evidence from adakitic granodiorites, gabbro-diorites and high-K granites in the Panxi arc belt

Neoproterozoic igneous rocks are widespread along the margins of the Yangtze Block and record key information on the tectonic evolution of the South China Craton. In this paper, we present a comprehensive study of zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Hf isotopes on the Neoproterozoic granodiorites, gabbro-diorites, and high-K granites in the western Yangtze Block. The granodiorites, gabbro-diorites and high-K granites were dated at 848 ± 5.3 Ma, 824 ± 5.1 Ma and 819 ± 1.5 Ma, respectively. The granodiorites have high Sr (567 ∼ 607 ppm) contents and Sr/Y (84.4 ∼ 88.0) ratios and low alkalis (Na2O + K2O = 4.3 ∼ 4.9 wt%) and Y (6.6 ∼ 7.1 ppm) and Yb (0.7 ∼ 0.9 ppm) contents, similar to typical adakitic rocks. They show low MgO (0.9 ∼ 1.3 wt%), Cr (15.2 ∼ 26.7 ppm) and Ni (3.6 ∼ 4.4 ppm) contents and positive εNd(t) (+1.7 to + 4.2) and εHf(t) (+3.0 to + 8.2) values. The parental magmas for granodiorites were generated by partial melting of thickened juvenile mafic lower crust. The gabbro-diorites are characterized by the enrichment in LREEs, significantly negative Nb, Ta and Ti anomalies, and positive εNd(t) (+2.2 to + 4.2) and εHf(t) (+5.8 to + 10.4) values. They are inferred to be derived from partial melting of lithospheric mantle modified by subduction-related fluids. The high-K granites display high Zr + Nb + Ce + Y (208 ∼ 366 ppm) and alkali (Na2O + K2O = 7.6 ∼ 8.4 wt%) contents, high 10000*Ga/Al (2.31 ∼ 2.64) ratios and zircon saturation temperatures, similar to A-type granites. We suggest that these granites were formed by the partial melting of felsic crustal materials under low pressure and high temperature conditions. The association of granodiorites, gabbro-diorites and high-K granites indicates Neoproterozoic oceanic slab subduction in the western Yangtze Block. We suggest that a subducting oceanic slab window can account for Neoproterozoic magmatism in the western Yangtze Block.

The Formation of the Jiaodong Gold Province

AbstractGold deposits in Jiaodong, termed Jiaodong‐type, are tectonically located in the southeastern margin of the North China Craton. Their major features are reviewed in this paper to highlight the differences between Jiaodong deposits and other genetic types of gold deposits. The mineralization was synchronized with asthenosphere upwelling indicated by syn‐ore OIB‐like mafic dike and large‐scale crustal thinning suggested by decrease of Sr/Y from pre‐ore to syn‐ore granites. Asthenosphere upwelling induced by the roll‐back of Paleo‐Pacific Plate drove partial melting of lithospheric mantle and devolatilization, which induced the release of the ore‐forming fluids. In concomitant with magmatic records, mineralization migrated from the western Jiaobei terrane (133–127 Ma) to the eastern Sulu orogenic belt (114–108 Ma), corresponding to the eastward roll‐back of Paleo‐Pacific Plate. Gold mineralization in Jiaodong formed in the transitions of ductile to brittle deformation, rapid to slow crustal uplift, and regional compression to extension. In the regional‐scale, the gold deposits in the Jiaobei terrane are mostly situated at intersections between NE‐trending faults and EW‐trending basement faults, and gold orebodies dominantly controlled by the lithologic contacts between Precambrian metamorphic rocks and Mesozoic granites. The mineralization was dominated by the disseminated‐veinlet ores related to quartz–sericite alteration in strong cataclasite‐breccia zone, with subsidiary thick quartz‐sulfide veins developed in secondary fault zones. The ore‐forming fluids belong to a H2O–CO2–NaCl±CH4system and show minor variations in salinity among different types of ore. Structure‐fluid feedback involving fluid‐rock reaction and hydrofracturing triggered the fluid phase separation and resultant gold deposition. The Jiaodong gold deposits are distinct from orogenic and intrusion‐related gold deposits in terms of tectonic setting, origin of ore‐forming fluids, and mechanism of gold deposition.

Late Jurassic syn-rift deposition in the Flemish Pass basin, offshore Newfoundland: Evidence for Tithonian magmatism and Appalachian-Variscan sediment sources from quantitative mineral and detrital zircon U–Pb-Hf isotope studies of Mizzen discovery strata

Quantitative mineral and detrital zircon U–Pb-Hf isotope studies of syn-rift sandstone units in the Mizzen F-09 delineation well, northern Flemish Pass basin, were conducted to test competing models for Grand Banks paleogeography and Late Jurassic development of the Newfoundland-Iberia magma-poor rift system. Bodhrán formation (informal nomenclature) strata yield ca. 145–155 Ma detrital zircon grain populations that indicate a late Tithonian (146 ± 1 Ma) maximum depositional age for syn-rift fluvial to tide-modulated fluvial strata in Mizzen F-09 and evidence for Grand Banks rift magmatism to have occurred throughout the Late Jurassic. Late Tithonian tectonic subsidence and magmatism in the northern Flemish Pass basin region were coincident with the onset of necking processes and extreme crustal thinning within the Newfoundland-Iberia rift system. The chondritic to superchondritic Hf isotope compositions of Late Jurassic detrital zircon grains indicate repeated partial melting of lithospheric mantle during rift development, perhaps along master detachment faults or transfer zone systems. Bodhrán formation strata have chemically unstable minerals (plagioclase, potassium feldspar, garnet, muscovite, rutile, staurolite) and late Paleozoic to Paleoarchean detrital zircon age populations that together show provenance from Appalachian-Variscan basement and cover assemblages. A positive correlation between the abundance of metamorphic minerals and unique-aged, mid-Permian (ca. 271–280 Ma) detrital zircon grains from post-Variscan igneous rocks indicate Iberian provenance contributions to Bodhrán formation strata. The new results from Mizzen F-09 are consistent with models for WNW-directed fluvial discharge into the northern Flemish Pass basin and sediment derivation from upper Paleozoic foreland basin or cover assemblages located west of Variscan front, some which may have been stripped off Beothuk Knoll, Flemish Cap, or adjacent highlands.

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.

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.

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.

Petrogenesis of the quartz diorite from the Lietinggang-Leqingla Pb-Zn-Fe-Cu-(Mo) deposit in southern Tibet: Implications for the genesis of a skarn-type polymetallic deposit in the Tibetan-Himalayan collisional orogen

The Lietinggang-Leqingla deposit is a representative skarn-type polymetallic deposit in the eastern of the central Lhasa subterrane. It comprises Pb-Zn and Fe-Cu-(Mo) mineralization that are part of the same metallogenic system. Garnet-skarn hosting Fe-Cu-(Mo) mineralization distributed in the contact zone between the Jubuzhari complex intrusions and carbonate strata of the Mengla Formation. The Pb-Zn mineralization is hosted in distal garnet-pyroxene and pyroxene-actinolite skarns, which formed in the strata of Mengla Formation. Here, we investigate quartz diorites (61.06 ± 0.93 Ma) that are related to Fe-Cu mineralization in the northern part of the mining area. The quartz diorites are characterized by low SiO2 (61.57–65.88 wt%), low K2O (2.53–3.07 wt%), and high total alkali (Na2O + K2O) contents (5.69–6.35 wt%). They are enriched in large-ion lithophile elements and light rare earth elements, but depleted in high field strength elements and heavy rare earth elements. The quartz diorites yield variably low (La/Yb)N ratios (4.29–7.07, the average value is 6.36) with moderately negative Eu anomalies (Eu/Eu* = 0.49–0.72). Furthermore, they exhibit a narrow range in (87Sr/86Sr)i ratios (0.706355–0.706807) and weakly negative εNd (t) (−2.08 to −1.57), and positive εHf (t) (+0.14 to +3.46) values. Based on the geochemical characteristics presented in this study and previously published data, we propose that the ore-forming rocks related to Fe-Cu and Pb-Zn mineralization were formed at the same time. The intermediate intrusions associated with Fe-Cu mineralization were derived from partial melting of lithospheric mantle mixed with the rejuvenated lower crustal melts, after which the mixed magma underwent minor fractional crystallization during upwelling. The felsic intrusions associated with Pb-Zn mineralization were formed by melting of mantle-derived basaltic magmas with old lower crust component contributions and which underwent extensive fractional crystallization.

Whole-Rock Geochemistry, Zircon U-Pb geochronology and Lu-Hf isotopic constraints on metasedimentary rocks (Paragneisses) in Stornes Peninsula, Larsemann Hills，East Antarctica

ABSTRACT Understanding the geological history of the Larsemann Hills and other exposures along the southeastern coast of Prydz Bay is essential for reconstructing the formation and evolution of the East Gondwana continent. This research involves analyses of the major elements, trace elements, REE, U-Pb and Lu-Hf isotopes of zircons from the metasedimentary rocks found in the Larsemann Hills. The results suggest that the provenance of the metasedimentary rocks within the Larsemann Hills is mainly Mesoproterozoic to early Neoproterozoic magmatic rocks. The age of the protolith of the metasedimentary rocks is between ~1019 and ~970 Ma. Our data show that at least two tectono-metamorphic events at ~913 Ma and ~552–555 Ma affected the metasedimentary rocks. Furthermore, Prydz Bay has been an integral whole since before the Palaeozoic era and was subsequently reactivated by Pan-African tectonic events. The thickened crust stretched and thinned, accompanied by partial melting of lithospheric mantle into the lower crust. The real Prydz Bay Pan-African collision orogenic belt should be located under the ice sheet east of the Grove Mountains and strikes close to north-northeast.

Multiple sources and magmatic evolution of the Late Triassic Daocheng batholith in the Yidun Terrane: Implications for evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau

Abstract Granitoids with diverse composition and tectonic settings provide important tools for exploring crustal evolution and regional geodynamic history. Here we present an integrated study using petrological, mineralogical, zircon U-Pb geochronological, whole-rock geochemical, and isotopic data on the Late Triassic Daocheng batholith in the Yidun Terrane with a view to understanding the petrogenesis of a compositionally diverse batholith and its implications for the evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau. The different lithological units of the batholith, including granodiorite, monzogranite, and quartz diorite, with abundant mafic microgranular enclaves in the granodiorite (MME I) and monzogranite (MME II), show identical crystallization ages of 218–215 Ma. The mineral assemblage and chemical composition of the granodiorite are identical to those of tonalitic-granodioritic melts generated under water-unsaturated conditions. The insignificant Eu anomalies and low magmatic temperatures indicate hydrous melting in the source. The relatively narrow range of whole-rock chemical and Sr-Nd isotopes, as well as the zircon trace element and Hf isotopic compositions of the granodiorite, suggest a homogeneous crustal source for the magma. Our modeling suggests that the rock was produced by 20–50% of lower crustal melting. The Daocheng monzogranites display more evolved compositions and larger variations in Sr-Nd-Hf isotopes than the granodiorite, which are attributed to assimilation and the fractional crystallization process. This is evidenced by the presence of metasedimentary enclave and inherited zircon grains with Neoproterozoic and Paleozoic ages, a non-cotectic trend in composition, and the trend shown by the modeling of initial 87Sr/86Sr ratios and Sr. The quartz diorites and MMEs showing composition similar to that of andesitic primary magma have high zircon εHf(t) values and are characterized by enrichment in LILEs and depletion of HFSEs. They were derived from the partial melting of lithospheric mantle that had been metasomatized by slab melts and fluids. The MMEs in both rocks display typical igneous texture and higher rare earth element (REE) and incompatible element concentrations than their host granites. The presence of fine-grained margins, acicular apatite, and plagioclase megacrysts suggests a magma mingling process. The overgrowth of amphibole around the pyroxene, quartz ocelli rimmed by biotite, and oscillatory zones of plagioclase are all indicative of chemical diffusion. Their enriched Sr-Nd isotopes imply isotopic equilibrium with the host granites. Based on a comparison with the coeval subduction-related magmatism, we propose that subduction and subsequent rollback of the Paleo-Tethys (Garzê-Litang Ocean) oceanic slab was the possible mechanism that triggered the diverse Triassic magmatism within the eastern Tibetan Plateau.

Geochemical studies of Sr-Nd-Pb-Hf isotopes on the Guojialing granite suite: Implications to region Au mineralization in the Jiaodong ore-cluster region

Based on the systematic elemental and isotope geochemical study on the Guojialing granite that is closely related to the gold mineralization in the Jiaodong ore-cluster region, further understandings have been made regarding its genetic mechanism, source material and gold mineralization conditions of the Guojialing granites. The (87Sr/86Sr)i values of Guojialing granite range from 0.7106 to 0.7120, and the εNd(t) from −18.1 to −13.2, respectively, which are similar to the initial SrNd isotopic compositions of those Late Jurassic-Early Cretaceous granites widely distributed in the Sulu orogenic belt, indicating similar sources of these intrusions in both Jiaodong and Su-Lu regions. The values of (206Pb/204Pb)i and(207Pb/204Pb)i of Guojialing granite are from 17.158–17.316, 15.453–15.478, respectively, indicating that the source of granites could be originated from mantle mixed with orogenic belt. The zircon Hf isotope of the Guojialing granite is decoupled from the Nd isotope of the whole rock, it has a zircon Hf model age(1979–3202 Ma) older than the full-rock Nd model age (1928 Ma). Compared to the full-rock Nd model age, the zircon Hf model age provides a more reliable age of crust-mantle differentiation and crust formation, suggesting that there is extensive crust deep-melting in the source area before the granitic magma activity, which was accompanied by strong Sm/Nd differentiation. Guojialing granite has similar characteristics to adakite, indicating that garnet is an important residual phase during magma formation. The formation of the Guojialing granite magma may be the partial melting of lithospheric mantle and thickened lower crust under eclogite facies, mixed with significant Neoarchaean crust or even Linglong granites when the magma upwelling. The Guojialing granite has high zircon Ce4+/Ce3+ ratios with the average values of 1151.7 and 811.4 respectively, indicating that the Guojialing granite was formed in a high oxygen fugacity environment, where sulfur is mainly present in the form of SO or SO2, which prevents the immiscibility of sulfides in the magma and avoids the removal of the sulfide metal elements. With crystallization differentiation, high oxygen fugitive magma will become a magma-hydrothermal fluid which is rich in sulfide metal elements, providing favorable material and environmental conditions for gold mineralization, thus favorably formed such giant gold deposit.

An Early Paleoproterozoic back-arc system along the southern margin of the Yinshan Block: Evidence from a newly-defined bimodal volcanic sequence in the Daqingshan Complex, Khondalite Belt

As one of the 2.1 to 1.9 Ga orogenic belts that welded the Columbia supercontinent, the Khondalite Belt in the North China Craton is a typical continent-continent collisional orogen that formed through the collision between the Yinshan and Ordos Blocks. Previous studies mostly focused on the collisional event in the Khondalite Belt but paid little attention to how the subduction system operated before the final closure of the ocean. To address this issue, we identified a series of interlayered meta-mafic and felsic rock assemblages in the Daqingshan Complex and implemented geochemical and geochronological analyses. Petrological and geochemical studies revealed that these rocks are bimodal and include plagioclase amphibolite (Group 1) and biotite plagiogneiss (Group 2). Geochemically, Group 1 samples show tholeiitic affinity, whereas Group 2 samples belong to the high-K calc-alkaline series. Geochemical data indicate that the protolith magma of Group 1 was most likely derived from the partial melting of lithospheric mantle with minor crustal contamination, whereas Group 2 rocks represent highly differentiated magma derived from the partial melting of ancient crustal materials. All the samples show depletion of HFSEs and enrichment of LILEs, indicative of a subduction-related magmatic arc environment. Zircon U-Pb dating results show that the protoliths of Group 1 samples yield crystallization ages of ∼2.47 Ga and metamorphic ages of 1.95 to 1.85 Ga, whereas the protoliths of Group 2 samples yield crystallization ages of ∼2.40 Ga and metamorphic ages of ∼1.85 Ga. Our new results and available geochemical, petrological, and isotopic data demonstrate that the bimodal volcanic sequence of the Daqingshan Complex was developed in a 2.47 to 2.40 Ga back-arc system along the southern margin of Yinshan Block. Subsequent collision between the Ordos and Yinshan Blocks resulted in the formation of the Khondalite Belt and final amalgamation of the Western Block between 1.95 and 1.85 Ga.

Late Permian–Triassic tectonic nature of the eastern Central Asian Orogenic Belt: Constraints from the geochronology and geochemistry of igneous rocks in the Bureya Massif

The late Paleozoic–early Mesozoic tectonic nature of the eastern Central Asian Orogenic Belt (CAOB) has been subject to debate, and the late Paleozoic–early Mesozoic magmatism in the Bureya Massif provides an opportunity to address this issue. We report new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data for late Paleozoic–early Mesozoic igneous rocks from the south of the Bureya Massif. These data provide insight into the petrogenesis of the igneous rocks, and constrain the late Paleozoic–early Mesozoic tectonic nature of the eastern CAOB. Zircon UPb ages indicate that the late Permian (ca. 255 Ma) and Middle–Late Triassic (ca. 216 Ma) magmatic events are identified in the south of the Bureya Massif. The late Permian igneous rocks consist mainly of bimodal rock suites, which include quartz monzodiorites, gneissic monzogranites, and A-type gneissic syenogranites. The Middle–Late Triassic igneous rocks are dominated by porphyritic syenogranites, and A-type granite porphyries, syenogranites, and gneissic macroporphyritic quartz syenites. The quartz monzodiorites have low SiO2 contents (53.9–56.1 wt%), high Mg# (56–57), and Cr (160–175 ppm), Co (25.2–27.8 ppm), and Ni (61.2–72.3 ppm) contents, and are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; e.g., K, Rb, Sr, and Ba). These characteristics, along with enriched zircon Hf compositions [εHf(t) = −8.31 to −0.44], suggest that their primary magma was derived from the partial melting of lithospheric mantle that had been metasomatized by fluids released from a fossil subducted slab. The granitoids have high SiO2, and low MgO contents, and are enriched in LILE, depleted in high field strength elements (HFSE), and yield εHf(t) values of −11.5 to +2.87 and two-stage model ages (TDM2) ages of 1968–1024 Ma. These characteristics suggest that they were derived from the partial melting of Paleoproterozoic–Mesoproterozoic accreted crustal material; however, these granitoids have distinct, heterogeneous geochemical compositions, precluding simple fractional crystallization of a single parental melt, and strongly suggesting that they were originated from distinct petrogenetic processes. The late Permian–Triassic bimodal igneous rock suites and A-type granites imply an extensional environment in the eastern CAOB.

Petrogenesis of the early Cretaceous intra-plate basalts from the Western North China Craton: Implications for the origin of the metasomatized cratonic lithospheric mantle

We present new bulk-rock 40Ar/39Ar age, major and trace elements and Sr-Nd-Hf isotopic data on the early Cretaceous intra-plate alkali basalts from the Western North China Craton (WNCC) to study the origin of the metasomatized cratonic lithosphere mantle. The age of these basalts is ~116 Ma. These basalts have elevated incompatible element abundance with high [La/Sm]N (2.80–4.56) and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.7062–0.7075, εNd(t) = −6.0 to −13.0 and εHf(t) = −8.3 to −17.4), being similar to the contemporary analogues from the Western North China Craton and Paleozoic kimberlites and mantle xenoliths. The WNCC basalts also show good correlations between ɛNd(t) and ɛHf(t), and high [La/Sm]N. All these geochemical observations are consistent with the interpretation that these basalts originated from partial melting of the lithospheric mantle that experienced melt metasomatism. Two types metasomatism melts are required to explain the geochemical characteristics of these rocks. The obvious negative Nb-Ta (compared with K)-Ti and positive Ba-Pb anomalies observed in these basalts further constrain that one of the metasomatic melts was derived from the subducted terrigenous sediment. Furthermore, the overall higher P/Nd, Nb/La and Nb/Th and lower Lu/Hf of basalts in the WNCC suggest that there is also contribution of low-F melts from asthenosphere mantle. Collectively, we suggest that the formation of the metasomatized lithosphere mantle beneath the WNCC is the process of metasomatic reaction between mantle peridotite and the melts of different origin to generate metasomatic veins containing amphibole/phlogopite. Partial melting of the metasomatic lithospheric mantle at 106–120 Ma in the WNCC was considered to be induced by thermal perturbation that was ultimately related to the breakoff of the subducted oceanic slab following the closure of the Mongolia-Okhotsk ocean.

Geochronology and geochemistry of low-grade metamorphic rocks from the Erdaowa Group and its significance on the tectonic evolution of the Paleoproterozoic Khondalite Belt, North China Craton

Khondalite Belt is located in the northwestern part of North China Craton, and is a typical continent-to-continent collisional orogen, along which the Yinshan Block collided with the Ordos Block at 1.95–1.85 Ga. However, this model was mainly relied on high-grade metamorphic records in the Khondalite Belt, whether the extensively exposed low-grade rocks were also involved in such collision remains poorly constrained. Erdaowa Group is an important lithological unit of the belt and consists of extensive low-grade metamorphic rock assemblages (i.e. greenschist to amphibolite facies), and provide crucial insights into understanding the issue. Based on petrological and geochemical characteristics, low-grade metamorphic rocks of Erdaowa Group show a large affinity to bimodal volcanic sequence characterized by basaltic and acidic endmembers. Meta-mafic rocks were derived from partial melting of lithospheric mantle with minor contamination of crustal materials, whereas meta-acidic rocks were generated from the middle-to-lower crust. Geochemical data of both rock assemblages show depletion of HFSE and enrichment of LILE, indicative of a subduction-related magmatic arc environment. U-Pb zircon dating results indicated that these rocks erupted at 2465 ± 42 Ma, and experienced metamorphism at 1940–1861 Ma. In combination with available petrological, geochemical and geochronological data, we infer that arc magmatism was developed at a back-arc basin area of a ~2.45 Ga active continental margin along southern Yinshan Block. Together with those high-grade metamorphic rocks, these low-grade rocks were also incorporated into the final collisional event at ~1.95–1.90 Ga.

Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting

The giant Central Asian Orogenic Belt is an extensive accretionary orogen, of which the Solonker suture, as a major regional suture, coincides closely with an early Permian paleobiogeographical boundary. This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane. Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian–Early Triassic, uncertainty remains because of a lack of typical collision-related features (e.g., high-grade regional metamorphism and well-developed fold–thrust structures) and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes (288–275 Ma) and Middle-Late Permian undeformed layered gabbros, strongly peraluminous granites, and I-type granites (265–254 Ma) in the Xinhure area along the northern margin of the North China Craton. The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton. In contrast, the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle–upper crust, respectively, triggered by asthenospheric upwelling. Therefore, a transition from an end-compressional to an extensional environment according to a transition from collision termination to post-collision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma. This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.

Geochronology, geochemistry, and Hf isotopic compositions of Triassic igneous rocks in the easternmost segment of the East Kunlun Orogenic Belt, NW China: implications for magmatism and tectonic evolution

ABSTRACT The voluminous Triassic igneous rocks in the East Kunlun Orogenic Belt (EKOB) are important for investigations of the tectonic evolution of the Paleo-Tethys Ocean. This study focused on the geochronology and petrogenesis of igneous rocks in the Elashan area with the aim of constraining the regional tectonic evolution. The 238 Ma syenogranites are characterized by high SiO2 contents and marked depletions in Sr and Eu. We conclude that they are highly fractionated I-type granites derived from partial melting of Mesoproterozoic lower crust with significant fractional crystallization. Comparing with contemporaneous granites and mafic rocks, the 236 Ma quartz monzodiorites have moderate MgO (1.5–1.6 wt.%) and SiO2 (61.0–61.9 wt.%) contents and εHf(t) values (–1.2 to +1.9). They were probably generated by the mixing of Mesoproterozoic lower-crust-derived and enriched lithospheric mantle-derived melts. The 230 Ma appinites have high MgO contents (3.8–6.1 wt.%) and enriched Hf isotopic compositions, and were likely derived from partial melting of lithospheric mantle that had been metasomatized by subduction fluids. We infer that these rocks were emplaced in syn-collisional slab break-off and the later initial post-collisional extensional setting between the BayanHar-Songpan Ganze and East Kunlun terranes. The basement composition, magmatism, and tectonic evolution of the Elashan area are similar to those of the eastern EKOB.

Tectonic evolution of the northeastern North China Craton: Constraints from geochronology and Sr–Nd–Hf–O isotopic data from Late Triassic intrusive rocks on Liaodong Peninsula, NE China

This paper reports new whole-rock major and trace element, SrNd isotopic, and zircon UPb and HfO isotopic data for Late Triassic intrusive rocks on Liaodong Peninsula, NE China, aiming to constrain the petrogenesis of these rocks in terms of lithospheric thinning in the northeastern North China Craton (NCC). New zircon UPb data indicate that the rocks were emplaced during the Late Triassic (220–211 Ma). Based on petrology, petrography and geochemistry, we divided these rocks into three groups. The Group 1 rocks, including hornblende gabbro, hornblende diorite and mafic vein, show low SiO2, Na2O + K2O and high MgO contents, with (87Sr/86Sr)i (0.7066–0.7071), εNd(t) (−10.6 to −5.7), εHf(t) (−16.6 to +1.5), and δ18O (7.06‰–7.82‰) values, indicating that they were formed by partial melting of lithospheric mantle with addition of lower continent crust materials. The Group 2 rocks, including syenites, display high SiO2, Na2O + K2O and low MgO contents, with (87Sr/86Sr)i (0.7064–0.7073), εNd(t) (−14.8 to −14.5), εHf(t) (−11.7 to −5.7), and δ18O (5.49‰–6.37‰) values, indicating they were originated through partial melting of lithospheric mantle with continent crust materials added, coupled by fractional crystallization of mafic minerals. The Group 3 rocks, including monzogranite and syenogranite, show high SiO2, Na2O + K2O and low MgO contents, with different isotopic signatures: the former shows (87Sr/86Sr)i (0.7085–0.7087), εNd(t) (−17.0 to −16.8), εHf(t) (−17.9 to −4.4) and δ18O (6.10‰–6.95‰) values. The latter shows higher εNd(t) (−9.0 to −8.1), εHf(t) (−3.4 to +1.2) and lower δ18O (5.52‰–7.29‰) values. Combined with the adakitic features of monzogranite and different two model ages, we believe they were derived from partial melting of ancient thickened lower crust and juvenile crust, respectively. Together with the spatial and temporal distribution of the Late Triassic rocks, we believe that these rocks were related to the collision between NCC and Yangtze Craton, and the initiation of lithospheric thinning of NCC in eastern segment began in Late Triassic.

Late Mesozoic tectonic evolution of the southern Great Xing'an Range, NE China: Evidence from whole-rock geochemistry, and zircon U[sbnd]Pb ages and Hf isotopes from volcanic rocks

The large-scaled Mesozoic volcanic activities in the Great Xing'an Range, northeastern (NE) China, have received extensive attention recently. However, the details of the tectonic setting and geodynamic processes of the Late Mesozoic time are still controversial. In this paper, we examined the geochronology and geochemistry of the Late Mesozoic volcanic rocks collected from the southern Great Xing'an Range, in order to constrain the tectonic evolution of the Great Xing'an Range in the Late Mesozoic time. The compiled data show that the Late Mesozoic volcanic rocks can be divided into three stages: Middle-Late Jurassic felsic volcanic rocks (174–148 Ma), Early Cretaceous intermediate and intermediate-felsic volcanic rocks (142–125 Ma, peak at 138 Ma) and Early Cretaceous felsic volcanic rocks (140–120 Ma, peak at 125 Ma). The Middle-Late Jurassic felsic volcanic rocks in the southern Great Xing'an Range are geochemically similar to A-type granite or highly fractionated granite, which have εHf(t) values of (−0.7 to +12.1) and Hf two-stage model (TDM2; 549–1714 Ma), indicating they are derived from the magma generated by partial melting of thickened juvenile lower crust and formed in a post-collisional extensional setting related to subduction of the Mongol–Okhotsk oceanic plate. The Early Cretaceous intermediate and intermediate-felsic volcanic rocks have the characteristics of active continental margin arc, which have εHf(t) values of −11.6 to +13.0 and TDM2 ages of 449–2670 Ma, suggesting that these rocks are the product of partial melting of lithospheric mantle. They are metasomatized by subduction-derived fluids or melts and related to the flat-slab subduction of the Mongol-Okhotsk oceanic plate beneath the Great Xing'an Range after the rapid closure of the Mongol-Okhotsk Ocean. The Early Cretaceous felsic volcanic rocks in the study are dominated by A-type rhyolite, which are associated with coeval A-type granites, metamorphic core complexes and rift basins, suggesting they formed in an extensional environment related to either the subduction of the Paleo-Pacific oceanic plate beneath the Eurasian continent or the massive upwelling of the asthenosphere following the collapse of the Mongol–Okhotsk oceanic plate.

Potassic/ultrapotassic intrusions at the southwestern margin of the Yangtze Craton, southwestern China: Petrogenesis and implications for the metal and fluid source of non-arc porphyry Cu–(Mo–Au) deposits

Magmatic rocks can reveal partial melting processes in the lower crust and mantle. Therefore, the petrogenesis of both ore-barren and ore-bearing magmatic rocks can provide key insights into the deep geological processes that control porphyry Cu–(MoAu) deposits (PCDs). Here, we investigate the petrogenesis of potassic/ultrapotassic igneous rocks in the southwestern Yangtze Craton (SWYC) to constrain their magmatic sources and assess their role in the formation of PCDs. These igneous rocks can be divided into three suites, ore-barren granite porphyry, lamprophyre, and ore-bearing quartz monzonite. UPb dating of zircons indicates that these potassic/ultrapotassic intrusions crystalized simultaneously between 35.4 and 33.6 Ma. The lamprophyres have low SiO2 contents and high MgO and K2O contents, exhibiting an ultrapotassic affinity. They are depleted in high field strength elements (HFSEs; Nb, Ta, Zr, Hf, and Ti) and enriched in large ion lithophile elements (LILEs; Rb, K, Sr, and Ba). The enriched Sr, Nd, and Hf isotopic compositions of the ore-barren lamprophyres suggest that they were derived from low-degree partial melting of lithospheric mantle that had been metasomatized by subduction-related fluids during the Proterozoic. The ore-barren granite porphyries have high SiO2 and K2O contents, with high-K calc-alkaline to shoshonitic characteristics. These rocks have low MgO, Cr, Ni, and Co contents. All samples exhibit high Sr/Y ratios, and highly fractionated rare earth elements (REEs) patterns with weak Eu anomalies. Combined with their low total REE contents (75–81 ppm) and εHf(t) values, which are similar to Neoproterozoic granulite xenoliths, we conclude that the ore-barren porphyries are adakites derived from thickened Neoproterozoic lower crust. The ore-bearing intrusions show variable SiO2 and high Na2O + K2O contents, plotting in the monzonite to quartz monzonite fields in a SiO2 vs Na2O + K2O diagram. These samples have much higher total REE contents (346–671 ppm) than the ore-barren intrusions. The homogeneous SrNd isotopic compositions and linear trends on Harker diagrams suggest that the magma that formed the ore-bearing rocks underwent fractional crystallization of mainly amphibole and accessory minerals. Water contents are estimated to have been 9–12 wt% at 0.8 GPa on the basis of the retention of plagioclase and low zircon saturation temperatures, which suggests that the primitive magmas of the ore-bearing intrusions were derived from water-fluxed melting rather than dehydration melting. The additional water, required to promote water-fluxed melting, was probably derived from underplated poassic/ultrapotassic mafic magmas. The high total REE contents, Sr/Y and (La/Yb)N ratios indicate that the primitive magmas of ore-bearing intrusions were probably derived from partial melting of potassic/ultrapotassic lower crust, formed by the underplating of mantle-derived potassic/ultrapotassic magma. These results, combined with previous research, show that mantle-derived potassic/ultrapotassic magmas can provide metals and fluids required for the formation of PCDs.