Crustal recycling through intraplate magmatism: Evidence from the Trans-North China Orogen

The North China Craton (NCC) preserves the history of crustal growth and craton formation during the early Precambrian followed by extensive lithospheric thinning and craton destruction in the Mesozoic. Here we present evidence for magma mixing and mingling associated with the Mesozoic tectonic processes from the Central NCC, along the Trans-North China Orogen, a paleo suture along which the Eastern and Western Blocks were amalgamated at end of Paleoproterozoic. Our investigations focus on two granitoids – the Chiwawu and the Mapeng plutons. Typical signatures for the interaction of mafic and felsic magmas are observed in these plutons such as: (1) the presence of diorite enclaves; (2) flow structures; (3) schlierens; (4) varying degrees of hybridization; and (5) macro-, and micro-textures. Porphyritic feldspar crystals show numerous mineral inclusions as well as rapakivi and anti-rapakivi textures. We present bulk chemistry, zircon U–Pb geochronology and REE data, and Lu–Hf isotopes on the granitoids, diorite enclaves, and surrounding basement rocks to constrain the timing of intraplate magmatism and processes of interaction between felsic and mafic magmas. Our LA-ICP-MS zircon U–Pb data show that the pophyritic granodiorite was emplaced at 129.7 ± 1.0 Ma. The diorite enclaves within this granodiorite show identical ages (128.2 ± 1.5 Ma). The basement TTG (tonalite–trondhjemite–granodiorite) gneisses formed at ca. 2.5 Ga coinciding with the major period of crustal accretion in the NCC. The 1.85 Ga age from zircons in the gabbro with positive Hf isotope signature may be related to mantle magmatism during post-collisional extension following the assembly of the Western and Eastern Blocks of the NCC along the Trans-North China Orogen. Our Hf isotope data indicate that the Neoarchean–Paleoproterozoic basement rocks were derived from complex sources of both juvenile magmas and reworked ancient crust, whereas the magma source for the Mesozoic units are dominantly reworked basement rocks. Our study provides a window to intraplate magmatism triggered by mantle upwelling beneath a paleosuture in the North China Craton.     

Keketuohai mafic–ultramafic complex in the Chinese Altai,NW China: Petrogenesis and geodynamic significance

Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409 ± 5 Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO₂, K₂O and Na₂O contents, negative covariations between P₂O₅, TiO₂ and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive ε_Nd(t) values (0 to + 2.7) and slight enrichments in light rare earth elements (e.g., La/Yb_N = 0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg# = 66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.     

Mesoproterozoic arc magmatism in SE India: Petrology,zircon U–Pb geochronology and Hf isotopes of the Bopudi felsic suite from Eastern Ghats Belt

The southern segment of the Eastern Ghats Mobile Belt (EGMB) in India was an active convergent margin during Mesoproterozoic, prior to the final collision in Neoproterozoic during the assembly of the Rodinia supercontinent. Here we present mineralogical, whole-rock geochemical, zircon U–Pb and Hf isotopic data from a granitoid suite in the Bopudi region in the EGGB. The granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites. The monzodiorite, locally porphyritic, contains K-feldspar megacrysts, plagioclase, quartz, biotite and ortho-amphibole. The presence of mantled ovoid megacrysts of alkali feldspar embaying early-formed quartz, and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites. The K-feldspar phenocrysts in the rapakivi granite are mantled by medium-grained aggregates of microcline (Ab₇ Or₉₃), which is compositionally equivalent to the rim of Kfs phenocryst and Pl (An_23–24 Ab₇₅). The geochemistry of both the granitoids shows arc-like features for REE and trace elements. LA-ICP-MS zircon analyses reveal ²⁰⁷Pb/²⁰⁶Pb ages of 1582 (MSWD = 1.4) for the rapakivi granite 1605 ± 3 Ma (MSWD = 3.9) for the monzodiorite. The zircons from all the granitoid samples show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. The zircons from the monzodiorite have a limited variation in initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.28171–0.28188, with εHf(t) values of −2.2 to +2.8. Correspondingly, their two-stage Hf isotope model ages (T_DM2) ranging from 2.15 to 2.47 Ga probably suggest a mixed source for the magma involving melting of the Paleoproterozoic basement and injection of subduction-related juvenile magmas. The prominent Mesoproterozoic ages of these granitoids suggest subduction-related arc magmatism in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic Rodinia assembly.     

Origin of the early Permian Wajilitag igneous complex and associated Fe–Ti oxide mineralization in the Tarim large igneous province,NW China

The Wajilitag igneous complex is part of the early Permian Tarim large igneous province in NW China, and is composed of a layered mafic–ultramafic intrusion and associated syenitic plutons. In order to better constrain its origin, and the conditions of associated Fe–Ti oxide mineralization, we carried out an integrated study of mineralogical, geochemical and Sr–Nd–Hf isotopic analyses on selected samples. The Wajilitag igneous rocks have an OIB-like compositional affinity, similar to the coeval mafic dykes in the Bachu region. The layered intrusion consists of olivine clinopyroxenite, coarse-grained clinopyroxenite, fine-grained clinopyroxenite and gabbro from the base upwards. Fe–Ti oxide ores are mainly hosted in fine-grained clinopyroxenite. Forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol%, indicating crystallization from an evolved magma. Reconstructed composition of the parental magma of the layered intrusion is Fe–Ti-rich, similar to that of the Bachu mafic dykes. Syenite and quartz syenite plutons have ε_Nd(t) values ranging from +1.4 to +2.9, identical to that for the layered intrusion. They may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization. Magnetites enclosed in olivines and clinopyroxenes have Cr₂O₃ contents higher than those interstitial to silicates in the layered intrusion. This suggests that the Cr-rich magnetite is an early crystallized phase, whereas interstitial magnetite may have accumulated from evolved Fe–Ti-rich melts that percolated through a crystal mush. Low V content in Cr-poor magnetite (<6600 ppm) is consistent with an estimate of oxygen fugacity of FMQ + 1.1 to FMQ + 3.5. We propose that accumulation of Fe–Ti oxides during the late stage of magmatic differentiation may have followed crystallization of Fe–Ti-melt under high fO₂ and a volatile-rich condition.     

Early to late Neoproterozoic magmatism and magma mixing–mingling in Sri Lanka: Implications for convergent margin processes during Gondwana assembly

The Sri Lankan fragment of Gondwana preserves the records of Neoproterozoic tectonothermal events associated with the final assembly of the supercontinent. Here we investigate a suite of magmatic rocks from the Wanni, Kadugannawa and Highland Complexes through geological, petrological, geochemical and zircon U–Pb and Lu–Hf isotopic techniques. The hornblende biotite gneiss, charnockites, metagabbro and metadiorites investigated in this study show geochemical features consistent with calc-alkaline affinity and subduction-related signature including LILE enrichment relative to HFSE coupled with distinct Nb–Ta depletion and weak negative Zr–Hf anomalies. The felsic suite falls in the volcanic arc granites (VAGs) field and the mafic suite shows island arc basalt affinity in tectonic discrimination plots, suggesting that the protoliths of the rocks were derived from arc-related magmas in a convergent margin setting. LA-ICPMS zircon U–Pb analyses show crystallization of charnockite and dioritic mafic magmatic enclave from the Highland Complex during ca. 565 and 576 Ma corresponding to bimodal magmatism. The diorite also contains metamorphic zircons of ca. 525 Ma. Hornblende–biotite gneiss from the Kadugannawa Complex shows protolith emplacement age at 973–980 Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic. The dioritic enclaves in these rocks are much younger, and form part of a deformed and metamorphosed dyke suite with emplacement ages of 559 Ma, broadly coeval with the bimodal magmatism in the Highland Complex at that time. The youngest group of zircons in this rock shows ages of 508 Ma, corresponding to the latest thermal event. A charnockite from this locality shows oldest group of zircons at 962 Ma, corresponding to emplacement age similar to that of the magmatic protolith of the hornblende biotite gneiss. This rock also shows zircon growth during repeated thermal events at 832 Ma, 780 Ma, 721 Ma and 661–605 Ma. The lower intercept age of 543 Ma marks the timing of collisional metamorphism. Charnockite from the Wanni Complex shows emplacement age at 1000 Ma, followed by thermal event at 570 Ma, the latter correlating with the bimodal magmatic event in the Highland Complex. The dioritic enclave within this charnockite shows an age of ca. 980 Ma, suggesting intrusion of mafic magma into the felsic magma chamber. Zircons in the diorite also record multiple zircon events during 950 to 750 Ma. Zircons in the Highland Complex charnockite possess negative εHf(t) values in the range − 6.7 to − 12.6 with T_DM^C of 2039–2306 Ma suggesting magma derivation through melting of Paleoproterozoic source. In contrast, the εHf(t) range of − 11.1 to 1.6 suggests a mixed source of both of older crustal and juvenile material. The εHf(t) values of − 4.5 to 4.5 and T_DM^C of 1546–1962 Ma for the hornblende biotite gneiss also shows magma derivation from mixed sources that included Paleoproterozoic components. The younger dioritic intrusive, however, has a more juvenile magma source as indicated by the mean εHf(t) value of 1.3. The associated charnockite shows a tight positive cluster of εHf(t) from 0.6 to 5.1, suggesting juvenile input. Charnockite from the Wanni Complex shows clearly positive εHf(t) values of up to 13.1, and T_DM^C in the range 937–1458 Ma suggesting much younger and depleted mantle source. The diorite enclave also has positive εHf(t) values with an average value of 8.5 and T_DM^C in the range of 709–1443 Ma clearly suggesting younger juvenile sources. The early and late Neoproterozoic bimodal suites are correlated to convergent margin magmatism associated with the assembly of Sri Lanka within the Gondwana supercontinent.     

Petrogenesis of granitoids in the Dewulu skarn copper deposit: implications for the evolution of the Paleotethys ocean and mineralization in Western Qinling,China

The Tethyan tectonic domain hosts numerous world-class mineral deposits. Among these, the Dewulu skarn copper deposit in Western Qinling, China belongs to the Paleotethys ore belt. The skarn and orebodies here occur as stratoids or lenses at the contact between the Triassic Dewulu intrusive complex and Permian marine clastic and carbonates. Alteration minerals include prograde skarns (garnet, diopside, wollastonite), plagioclase, hornblende, actinolite, tremolite, epidote, chlorite, calcite, quartz and sericite. The main ore types include early disseminated skarn-type replacement orebodies and late-stage quartz-sulfide veins. Chalcopyrite is the major ore mineral, along with pyrite, bornite and sphalerite. The Dewulu intrusive complex comprises quartz diorite, quartz diorite porphyry and dioritic mafic microgranular enclaves (MME). The MMEs are spheroidal in shape, and have igneous mineral assemblages, acicular apatites, complex oscillatory zoned plagioclase and quartz megacrysts surrounded by mafic minerals. The MMEs are metaluminous and calc-alkaline to high-K calc-alkaline, and possess relatively high Ni, Cr and MgO contents and Mg^# values. They display sub-parallel patterns in trace element spider diagrams and rare earth element (REE) plots. They are also characterized by the enrichment of Rb, U and Th, depletion of Ba, Sr, Nb and Ta and negative Eu anomaly. Zircon LA-ICP-MS U–Pb dating of the dioritic MME yields an age of 247.0 ± 2.2 Ma, coeval with the host quartz diorite, quartz diorite porphyry and ore-related sericite ⁴⁰Ar/³⁹Ar plateau ages within analytical uncertainties. Oxygen fugacity estimated from trace element compositions of zircons from the dioritic MME shows FMQ ± 3.3. The zircons have negative ε_Hf(t) values in a range of − 8.0 to − 3.3, corresponding to two-stage model ages ranging from 1.48 to 1.78 Ga. The integrated data from petrology, geochronology and bulk geochemistry suggest that the Early Triassic granitoids associated with Cu skarn mineralization at Dewulu were products of arc magmatism and involved magma mixing in an active continental margin setting. The magma was sourced through partial melting of enriched sub-continental lithospheric mantle that had been previously modified by slab-derived melt during the continuous northward subduction of the Paleotethys oceanic slab.     

Geochemical characteristics of the Kuh-e Dom intrusion,Urumieh–Dokhtar Magmatic Arc (Iran): Implications for source regions and magmatic evolution

The Kuh-e Dom Pluton is located along the central northeastern margin of the Urumieh–Dokhtar Magmatic Arc, spanning a wide range of compositions from felsic rocks, including granite, granodiorite, and quartz monzonite, through to intermediate-mafic rocks comprising monzonite, monzodiorite, diorite, monzogabbro, and gabbro. The Urumieh–Dokhtar Magmatic Arc forms a distinct linear magmatic complex that is aligned parallel with the orogenic suture of the Zagros fold-thrust belt. Most samples display characteristics of metaluminous, high-K calc-alkaline, I-type granitoids. The initial isotopic signatures range from εNd (47 Ma) = −4.77 to −5.89 and ⁸⁷Sr/⁸⁶Sr(i) = 0.7069 to 0.7074 for felsic rocks and εNd (47 Ma) = −3.04 to −4.06 and ⁸⁷Sr/⁸⁶Sr(i) = 0.7063 to 0.7067 for intermediate to mafic rocks. This geochemical and isotopic evidence support a mixed origin for the Kuh-e Dom hybrid granitoid with a range of contributions of both the crust and mantle, most probably by the interaction between lower crust- and mantle-derived magmas. It is seem, the felsic rocks incorporate about 56–74% lower crust-derived magma and about 26–44% of the enriched mantle-derived mafic magma. In contrast, 66–84% of the enriched mantle-derived mafic magma incorporates 16–34% of lower crust-derived magma to generate the intermediate-mafic rocks. According to the differences in chemical composition, the felsic rocks contain a higher proportion of crustal material than the intermediate to mafic ones. Enrichment in LILEs and depletion in HFSEs with marked negative Nb, Ba, and Ti anomalies are consistent with subduction-related magmatism in an active continental margin arc environment. This suggestion is consistent with the interpretation of the Urumieh–Dokhtar Magmatic Arc as an active continental margin during subduction of the Neotethys oceanic crust beneath the Central Iranian microcontinent.     

Mineral chemistry and genesis of the Permian Cihai and Cinan magnetite deposits,Beishan, NW China

Cihai and Cinan are Permian magnetite deposits related to mafic-ultramafic intrusions in the Beishan region, Xinjiang, NW China. The Cihai mafic intrusion is dominantly composed of dolerite, gabbro and fine-grained massive magnetite ore, while gabbro, pyrrhotite + pyrite-bearing clinopyroxenite and magnetite ore comprise the major units in Cinan. Clinopyroxene occurs in both deposits as 0.1–2 mm in diameter subhedral to anhedral grains in dolerite, gabbro and clinopyroxenite. High FeO contents (11.7–28.9 wt%), low SiO₂ (43.6–54.3 wt%) and Al₂O₃ contents (0.15–6.08 wt%), and low total REE and trace element contents of clinopyroxene in the Cinan clinopyroxenite imply crystallization early, at high pressure. This clinopyroxene is FeO-rich and Si and Ti-poor, consistent with the clinopyroxene component of large-scale Cu-Ni sulfide deposits in the Eastern Tianshan and Panxi ares, as well as Tarim mafic intrusion and basalt, implying the Cinan mafic intrusion and sulfide is related to tectonic activity in the Tarim LIP. The similar mineral chemistry of clinopyroxene, apatite and magnetite in the Cihai and Cinan gabbros (e.g., depleted LREE, negative Zr, Hf, Nb and Ta anomalies in clinopyroxene, lack of Eu anomaly in apatite and similarity of oxygen fugacity as indicated by V in magnetite), indicate similar parental magmatic characteristics. Mineral compositions suggest a crystallization sequence of clinopyroxenite/with a small amount of sulfide – gabbro – magnetite ore in the Cinan deposit, and magnetite ore – gabbro – dolerite in Cihai. The basaltic magma was emplaced at depth, with magnetite segregation (and formation of the Cinan magnetite ores) occurring in relatively low fO₂ conditions, after clinopyroxenite and gabbro fractional crystallization. The evolved Fe-rich basaltic magma rapidly rose to intermediate or shallow depths, forming an immiscible Fe-Ti oxide magma as fO₂ increased and leaving a Fe-poor residual magma in the chamber. The residual magmas was emplaced at different levels in the crust, forming the Cihai gabbro and dolerite, respectively. Finally, the immiscible Fe-Ti oxide magma was emplaced into the earlier formed dolerite because of late magma pulse uplift, resulting in a distinct boundary between the magnetite ores and dolerite.     

Geochronology and geochemical characteristics of the Dongping ore-bearing granite,North China: Sources and implications for its tectonic setting

The Dongping gold deposit, located in Chongli County (Hebei Province) about 200 km northwest of Beijing, is one of the largest gold-producing areas along the northern margin of the North China Craton. It is located in the of Shuiquangou alkaline igneous complex of Middle Devonian age (394.3 ± 3.2 Ma), composed chiefly of highly alkaline syentite and quartz syenites. This study reveals the age of the Carboniferous in the deposit at 351.7 ± 2.8 Ma (MSWD = 1.9). The Dongping deposit is locally hosted in Cretaceous (~143 ± 1 Ma) alkali granites that intruded the older and the gold mineralization is closely associated genetically with this event. Hydrothermal zircons in the alkali granites have Th/U ratios mostly ranging between 0.01 and 0.7 indicating oscillatory zoning. A few grains with high Th/U ratios (1.31–2.07) may be from metamorphic domains. Negative εHf(t) values of the zircon mainly range between −19.75 and −16.93, suggesting that they originated principally by the melting of recycled continental crust. Less abundant zircons with εHf(t) ranging from −25.76 to −23.46, with Hf model ages (T_DM2) of 2.54 to 2.67 Ga, (mainly 2.2 to 2.3 Ga) suggest that recycled Neoarchean basement was also present in the source region. The Devonian syenites and quartz syenites have T_DM1 ages ranging from 1.96 to 2.08 Ga. Zircons from these rocks have εHf(t) values of −11.9 to −18.9. Certain zircons from the gold-bearing granite of Paleozoic age have an initial ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.281816 to 0.282058 and 0.282147 to 0.282348, reflecting a homogenous distribution of hafnium isotopes typical of magmatic sources. The T_DM1 and T_DM2 of the latest intrusion varying 1.33 to 1.59 Ga and 1.72 to 2.11 Ga respectively, indicating that the Neoproterozoic to Mesoproterozoic rocks of this area are an important source for the younger magma which are important to forming ore deposits. The TDM₂ indicate that the magma may be derived from a very old crustal basement (~2.67 Ga) in the northern margin of North China Craton by partial melting.     

Multi-stage granitic magmatism during exhumation of subducted continental lithosphere: Evidence from the Wulong pluton,South Qinling

Detailed petrology and zircon U–Pb dating data indicate that the Wulong pluton is a zoned granitic intrusive, formed from successive increments of magmas. An age range of at least 30 Ma is recorded from the 225–235 Ma quartz diorite on the pluton margin, the ca. 218 Ma granodiorite in the intermediate zone, and the ca. 207 Ma monzogranite at the pluton center. All the granitoids display evolved Sr–Nd–Pb isotopic compositions, with ⁸⁷Sr/⁸⁶Sr(i) of 0.7044–0.7062, unradiogenic Nd (ε_Nd(t) values of − 6.1 to − 3.0, Nd model ages of 1.1–1.3 Ga, and moderately radiogenic Pb compositions (²⁰⁶Pb/²⁰⁴Pb(i) = 17.500–17.872, ²⁰⁷Pb/²⁰⁴Pb(i) = 15.513–15.549, ²⁰⁸Pb/²⁰⁴Pb(i) = 37.743–38.001), in combination with variations in zircon Hf isotopic compositions (with ε_Hf(t) values in each stage span 12 units) and the Hf isotopic model ages of 800–1600 Ma. These features suggest that the granitoids might have been derived from the reworking of an old lower crust, mixed with Paleozoic and Proterozoic materials. The rocks also display an adakitic affinity with Sr (479–973 ppm), high Sr/Y ratios (mostly > 60) and negligible Eu anomalies (Eu/Eu* = 0.78–0.97) but low Rb/Sr ratios, low Y (4.6–17 ppm), HREE (Yb = 0.95–1.7 ppm), Yb/Lu (6–7) and Dy/Yb (1.9–2.4) ratios, suggesting the absence of plagioclase and presence of garnet + amphibole in their residue. Considering a large gap among their crystallization ages, we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization. Mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition, and are metaluminous and enriched in LREE and LILEs, but are depleted in HFSE, and display an evolved Sr–Nd–Pb isotopic composition, suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere, which was metasomatized by adakitic melts and fluids from a subducted continental crust.In combination with the results of the Triassic ultra-high pressure metamorphic rocks in the Dabie orogenic belt, we apply a model involving the exhumation of subducted continental crust to explain the formation of the Wulong pluton. At the first stage, a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma, characterized by low SiO₂ (60.65–63.98 wt.%) and high TiO₂ (0.39–0.86 wt.%). The magma subsequently interacted with mantle peridotite, leading to high Mg# (57–67) and the metasomatism of the overriding mantle wedge. At the second stage, an asthenosphere upwelling that was probably caused by slab break-off at ca. 220 Ma melted the enriched sub-continental lithospheric mantle (SCLM) to produce mafic magmas, represented by the mafic enclaves that are hosted in the quartz and granodiorite, resulting in the partial melting of the shallower subducted crust, and generating the granodiorite that is distinguished by high SiO₂ (69.16–70.82 wt.%), high Al₂O₃ (15.33–16.22 wt.%) and A/CNK values (mostly > 1.05). At the third stage, the final collapse of the Triassic Qinling–Dabie Orogenic Belt at ca. 215–205 Ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite, which is characterized by high SiO₂ (67.68–70.29 wt.%), low TiO₂ (mostly < 0.35 wt.%) and high Sr/Y ratios of 86–151.     

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite,central Eastern Desert,Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P₂O₅ anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ～830 °C/～5 kbar. The crystallization conditions of the quartz diorites were estimated at ～570 °C/～2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.     

Mineral chemistry of high-Mg diorites and skarn in the Han-Xing Iron deposits of South Taihang Mountains,China: Constraints on mineralization process

The Han-Xing region is located in the south Taihang Mountains (TM) in the central part of the North China Craton, and is an important iron producing area. The iron deposits in this region are of skarn type, related to an Early Cretaceous high-Mg diorite complex, including gabbro diorite, hornblende diorite, diorite, diorite porphyrite, and monzonite. In this study we report the detailed mineral chemistry of the high-Mg diorites and skarn rocks. The olivine in the gabbro diorite shows chemical composition similar to that in mantle peridotite xenoliths. Clinopyroxene in the gabbro diorite is dominantly augite, with only minor diopside, whereas the clinopyroxenes in the diorite and monzonite are diopside. Amphiboles in the high-Mg diorites show compositional range from magnesiohornblende to magnesiohastingsite, with minor pargasite and tschermakite. Most plagioclase in the high-Mg diorite is andesine and oligoclase. The magnesio-biotite in gabbro diorites shows chemical characteristics of re-equilibrated primary biotites and those in calc-alkaline rocks. In the diorite and diorite porphyrite, plagioclase shows complex chemical zoning. Clinopyroxene and garnet in skarn rocks show varying FeO contents, the former containing low FeO (< 9 wt.%) and occurring as the major skarn mineral in large-scale iron deposits, and the latter within small-scale iron deposits with high FeO (mostly > 25 wt.%) content. We computed the pressure, temperature, oxygen fugacity and water contents based on the mineral chemistry of amphibole and biotite. Based on the results, the magma crystallization can be divided into two stages, one within the deep magma chamber, forming clinopyroxene, amphibole and plagioclase phenocrysts; the other after emplacement, forming the rim of phenocrysts and matrix minerals. The magma during the early stage shows high temperature (~ 900 °C–950 °C), pressure (~ 300 MPa–500 MPa), relatively high logfO₂ (NNO–NNO + 2), and H₂O content in melt (4%–8%). During the late stage, the magma temperature dropped to about 750 °C, and pressure came down to less than 100 MPa, with the logfO₂ rising to NNO + 1–NNO + 2.The zoning of amphibole and plagioclase records the process of magma mixing and crystallization, with injection of mafic magma into the felsic magma chamber. The relatively high logfO₂ and H₂O content inhibited partitioning of iron into mafic minerals and favored concentration of Fe in the melt. Iron ore precipitation occurred when the magma was emplaced at shallow level, and was principally controlled by the chemical composition of carbonate wall rocks. The high logfO₂, Fe^3 + rich ore-forming fluid generated andradite and clinopyroxene when it reacted with limestone and dolomitic limestone respectively.     

Stages and conditions of metamorphism of mafic granulites in the Early Precambrian complex of the Angara–Kan terrane (southwestern Siberian Craton)

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~ 900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~ 1.77 Ga) and two-pyroxene granulites (~ 1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~ 1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of ¹⁷⁶Hf/¹⁷⁷Hf values for the core and rims. The positive ε_Hf values (up to + 6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~ 1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).     

Early Carboniferous sub- to mid-alkaline magmatism in the Eastern Sierras Pampeanas,NW Argentina: A record of crustal growth by the incorporation of mantle-derived material in an extensional setting

A recently discovered granitic intrusion at Cerro La Gloria in western Sierra de Famatina (NW Argentina) is representative of sub- to mid-alkaline Carboniferous magmatism in the region. The main rock type consists of microcline, quartz and plagioclase, with amphibole, magnetite, ilmenite, biotite, epidote, zircon, allanite and sphene as accessory minerals. We report a U–Pb zircon SHRIMP age for the pluton of 349 ± 3 Ma (MSWD = 1.1), i.e., Tournaisian. Whole-rock chemical composition and Nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust (εNdt between − 0.58 and + 0.46 and T_DM values of about 1.1 Ga). The pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back-arc basalts. Coeval, Early Carboniferous A-type granites occur farther east in the Sierras Pampeanas, probably generated during lithospheric stretching. Overall, the Early Carboniferous granitic rocks show a west-to-east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west. Based on the observed patterns of geochronology, geochemistry and field relationships we suggest that A-type magma genesis in the Eastern Sierras Pampeanas was linked to an Andean-type margin where the lithospheric mantle played a role in its generation.     

Petrology,geochemistry and geochronology of the magmatic suite from the Jianzha Complex,central China: Petrogenesis and geodynamic implications

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO^tot and Cr₂O₃ of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)_N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu¹ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε_Nd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.     

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.     

Archaean to Palaeoproterozoic high-grade evolution of the Belomorian eclogite province in the Gridino area,Fennoscandian Shield: Geochronological evidence

The Belomorian eclogite province was repeatedly affected by multiple deformation episodes and metamorphism under moderate to high pressure. Within the Gridino area, high pressure processes developed in a continental crust of tonalite–trondhjemite–granodiorite (TTG) affinity that contains mafic pods and dykes, in which products of these processes are most clearly evident. New petrological, geochemical and geochronological data on mafic and felsic rocks, including PT-estimates, mineral chemistry, bulk rock chemistries, REE composition of the rocks and zircons and U–Pb and Lu–Hf geochronology presented in the paper make it possible to reproduce the magmatic and high-grade metamorphic evolution in the study area. In the framework of the extremely long-lasting geologic history recorded in the Belomorian province (3–1.7 Ga), new geochronological data enabled us to define the succession of events that includes mafic dyke emplacement between 2.87 and 2.82 Ga and eclogite facies metamorphism of the mafic dykes between ~ 2.82 and ~ 2.72 Ga (most probably in the time span of 2.79–2.73 Ga). The clockwise PT path of the Gridino association crosses the granulite- and amphibolite-facies PT fields during the time period of 2.72 Ga to 2.64 Ga. A special aspect of this work concerns the superposed subisobaric heating (thermal impact) with an increase in the temperature to granulite facies conditions at 2.4 Ga. Later amphibolite facies metamorphism occurred at 2.0–1.9 Ga. Our detailed geochronological and petrological studies reveal a complicated Mesoarchaean–Palaeoproterozoic history that involved deep subduction of the continental crust and a succession of plume-related events.     

Geologic and geochemical insights into the formation of the Taiyangshan porphyry copper–molybdenum deposit,Western Qinling Orogenic Belt,China

Taiyangshan is a poorly studied copper–molybdenum deposit located in the Triassic Western Qinling collisional belt of northwest China. The intrusions exposed in the vicinity of the Taiyangshan deposit record episodic magmatism over 20–30 million years. Pre-mineralization quartz diorite porphyries, which host some of the deposit, were emplaced at 226.6 ± 6.2 Ma. Syn-collisional monzonite and quartz monzonite porphyries, which also host mineralization, were emplaced at 218.0 ± 6.1 Ma and 215.0 ± 5.8 Ma, respectively. Mineralization occurred during the transition from a syn-collisional to a post-collisional setting at ca. 208 Ma. A barren post-mineralization granite porphyry marked the end of post-collisional magmatism at 200.7 ± 5.1 Ma. The ore-bearing monzonite and quartz monzonite porphyries have a ε_Hf(t) range from − 2.0 to + 12.5, which is much more variable than that of the slightly older quartz diorite porphyries, with T_DM2 of 1.15–1.23 Ga corresponding to the positive ε_Hf(t) values and T_DM1 of 0.62–0.90 Ga corresponding to the negative ε_Hf(t) values. Molybdenite in the Taiyangshan deposit with 27.70 to 38.43 ppm Re suggests metal sourced from a mantle–crust mixture or from mafic and ultramafic rocks in the lower crust. The δ³⁴S values obtained for pyrite, chalcopyrite, and molybdenite from the deposit range from + 1.3‰ to + 4.0‰, + 0.2‰ to + 1.1‰, and + 5.3‰ to + 5.9‰, respectively, suggesting a magmatic source for the sulfur. Calculated δ¹⁸O_fluid values for magmatic K-feldspar from porphyries (+ 13.3‰), hydrothermal K-feldspar from stockwork veins related to potassic alteration (+ 11.6‰), and hydrothermal sericite from quartz–pyrite veins (+ 8.6 to + 10.6‰) indicate the Taiyangshan deposit formed dominantly from magmatic water. Hydrogen isotope values for hydrothermal sericite ranging from − 85 to − 50‰ may indicate that magma degassing progressively depleted residual liquid in deuterium during the life of the magmatic–hydrothermal system. Alternatively, δD variability may have been caused by a minor amount of mixing with meteoric waters. We propose that the ore-related magma was derived from partial melting of the ancient Mesoproterozoic to Neoproterozoic middle to lower continental crust. This crust was likely metasomatized during earlier subduction, and the crustal magmas may have been contaminated with lithospheric mantle derived magma triggered by MASH (e.g., melting, assimilation, storage, and homogenization) processes during collisional orogeny. In addition, a significant proportion of the metals and sulfur supplied from mafic magma were simultaneously incorporated into the resultant hybrid magmas.     

Petrogenesis and tectonic significance of the ∼ 850 Ma Gangbian alkaline complex in South China: Evidence from in situ zircon U–Pb dating,Hf–O isotopes and whole-rock geochemistry

The Gangbian alkaline complex in the southeastern Yangtze Block (South China) is composed of Si-undersaturated pyroxene syenites and Si-saturated to -oversaturated syenites and quartz monzonites. SIMS zircon U–Pb analyses indicate that the complex was emplaced at 848 ± 4 Ma, during a previously-recognized interval of magmatic quiescence between the ca 1.0–0.89 Ga Sibaoan orogenic magmatism and the ca 0.83–0.78 Ga magmatic flare-up. The Gangbian rocks are characterized by wide, coherent variations in major and trace elements (SiO₂ = 47.6–68.4%, K₂O + Na₂O = 4.5–10.5%, K₂O/Na₂O = 0.4–1.2, MgO = 1.2–8.5%, Cr = 4.5–239 ppm, and Ni = 4.5–143 ppm) and by enrichment in LIL and LREE and depletion in Nb, Ta and P in trace element spidergrams. Their whole-rock εNd(T) (? 6.5 to ? 0.4) and εHf(T) (? 10.7 to 0.4) are positively correlated, suggesting involvement of both metasomatized mantle and continental crust materials in their genesis. In situ zircon Hf–O isotopic measurements for the most evolved quartz monzonite sample yield a binary mixing trend between the mantle- and supracrustal-derived melts. It is suggested that the pyroxene syenites were derived by partial melting of metasomatized, phlogopite-bearing lithospheric mantle, and the parental magma experienced extensive fractionation of pyroxene and olivine associated with varying degrees of crustal contamination. Subsequent fractional crystallization of hornblende and minor amounts of plagioclase from the alkali basaltic magmas, accompanied by crustal contamination, produced the Si-saturated to -oversaturated syenites and quartz monzonites. These ca. 0.85 Ga alkaline rocks and neighboring contemporaneous dolerite dykes are the products of the anorogenic magmatism after the Sibao Orogeny. They post-date the final amalgamation between the Yangtze and Cathaysia Blocks, most likely manifesting the initial rifting of South China within the Rodinia supercontinent.     

In-situ LA-ICPMS trace elements and U–Pb analysis of titanite from the Mesozoic Ruanjiawan W–Cu–Mo skarn deposit,Daye district,China

In this paper, we present U–Pb ages and trace element compositions of titanite from the Ruanjiawan W–Cu–Mo skarn deposit in the Daye district, eastern China to constrain the magmatic and hydrothermal history in this deposit and provide a better understanding of the U–Pb geochronology and trace element geochemistry of titanite that have been subjected to post-crystallization hydrothermal alteration. Titanite from the mineralized skarn, the ore-related quartz diorite stock, and a diabase dike intruding this stock were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Titanite grains from the quartz diorite and diabase dike typically coexist with hydrothermal minerals such as epidote, sericite, chlorite, pyrite, and calcite, and display irregular or patchy zoning. These grains have low LREE/HREE and high Th/U and Lu/Hf ratios, coupled with negative Eu and positive Ce anomalies. The textural and compositional data indicate that titanite from the quartz diorite has been overprinted by hydrothermal fluids after being crystallized from magmas. Titanite grains from the mineralized skarn are texturally equilibrated with retrograde skarn minerals including actinolite, quartz, calcite, and epidote, demonstrating that these grains were formed directly from hydrothermal fluids responsible for the mineralization. Compared to the varieties from the quartz diorite stock and diabase dike, titanite grains from the mineralized skarn have much lower REE contents and LREE/HREE, Th/U, and Lu/Hf ratios. They have a weighted mean ²⁰⁶Pb/²³⁸U age of 142 ± 2 Ma (MSWD = 0.7, 2σ), in agreement with a zircon U–Pb age of 144 ± 1 Ma (MSWD = 0.3, 2σ) of the quartz diorite and thus interpreted as formation age of the Ruanjiawan W–Cu–Mo deposit. Titanite grains from the ore-related quartz diorite have a concordant U–Pb age of 132 ± 2 Ma (MSWD = 0.5, 2σ), which is 10–12 Ma younger than the zircon U–Pb age of the same sample and thus interpreted as the time of a hydrothermal overprint after their crystallization. This hydrothermal overprint was most likely related to the emplacement of the diabase dike that has a zircon U–Pb age of 133 ± 1 Ma and a titanite U–Pb age of 131 ± 2 Ma. The geochronological results thus reveal two hydrothermal events in the Ruanjiawan deposit: an early one forming the Wu–Cu–Mo ores related to the emplacement of the quartz diorite stock and a later one causing alteration of the quartz diorite and its titanite due to emplacement of diabase dike. It is suggested that titanite is much more susceptible to hydrothermal alteration than zircon. Results from this study also highlight the utilization of trace element compositions in discriminating titanite of magmatic and hydrothermal origins, facilitating a more reasonable interpretation of the titanite U–Pb ages.