Depletion In High Field Strength Elements Research Articles

Neoproterozoic tectonic transition in the South Qinling Belt: New constraints from geochemistry and zircon U–Pb–Hf isotopes of diorites from the Douling Complex

The Douling Complex, which represents the oldest rock unit found in the South Qinling Belt (SQB), was intruded by voluminous Neoproterozoic magmatic rocks. These Neoproterozoic magmatic rocks can provide important constraints on the tectonic evolution and crust growth of the SQB, but are still poorly studied. In this contribution, we present geochemical, in-situ zircon U–Pb–Hf isotope and trace element data of Neoproterozoic granodiorite and quartz diorite from three (i.e., Gangou, Sanpinggou and Fenzishan) plutons of the Douling Complex. Zircon U-Pb isotopic dating shows that these granodiorite and quartz diorite have ages in a small range of 693–719 Ma and zircon εHf(t) values from −12.3 to 12.6. The granodiorite and quartz diorite samples have intermediate SiO2 (55.05–66.50 wt%), K2O (1.23–3.00 wt%) and high Mg-number (Mg# = 46–67). They display enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), indicating that their mantle source was modified by subducted materials. Our new data, integrated with previous results, indicate that the Neoproterozoic magmatism in the SQB had three major episodes at ca. 630, 750 and 860 Ma, respectively. Zircon Hf isotopic data of these rocks reveal that the magmatic sources involved both Meso- to Neoproterozoic juvenile crust and reworked Neoarchean to Paleoproterozoic components. An abrupt increase in the contribution of juvenile crust and depleted mantle relative to old crust occurred in 750 to 630 Ma. In combination with the occurrence of synchronous A-type granite in the region, this geochemical shift indicates the tectonic shift from the convergence to divergence in the SQB area.

Age and origin of subvolcanic rocks from NE Iran: Link between magmatic “flare-up” and mineralization

Paleogene igneous rocks from ∼600 km Quchan-Sabzevar-Torud magmatic belt include a thick pile of volcanic and pyroclastic rocks which intruded by younger felsic-mafic plutons. Various types of mineralization including Iron Oxide-Copper-Gold (e.g., Firouzeh mine) and porphyry Cu-Au deposits (e.g., Jalambadan mine) are associated with the Quchan-Sabzevar magmatism. In this study, we describe new zircon U-Pb ages and geochemical-isotopic data of the subvolcanic rocks from near the Firouzeh mine. The Firouzeh subvolcanic rocks consist of (quartz-bearing) monzosyenites, monzodiorites and monzonites. These rocks have typical calc-alkaline signature and are mainly metaluminous in nature. Subvolcanic rocks display enrichment in Light Rare Earth Elements (LREEs) with negative Eu anomaly. Enrichment in Large Ion Lithophile Elements (LILEs) and depletion in High-Field Strength Elements (HFSEs) are geochemical characteristics of these rocks. The Firouzeh volcanic rocks also display calc-alkaline signature and are metaluminous to peraluminous. Volcanic rocks show both enrichment in LREEs and LILEs, associated with negative Eu anomaly. Zircon U-Pb indicates ages of 43.2 ± 0.4, 42.1 ± 0.4 and 41.8 ± 0.4 Ma for monzosyenites, monzodiorites and monzonites respectively. Zircon epsilon Hf(t) shows average values of −1.49 for monzonites, +9.07 for monzodiorites and −1.06 for monzosyenites. The Hf model ages for these rocks are in the range of 850–730, 270–180 and 3150–450 Ma, respectively. Inherited zircons are abundant in monzonites and have variable Hf isotope values. The wide range of zircon εHf(t) values and abundance of inherited/xenocrystic zircons suggest a multiple source(s) for the generation of the Firouzeh subvolcanic rocks, including a mantle melt and an old crustal component. Xenocrystic zircons indicate complex crustal components. We suggest the NE Iran subvolcanic rocks including the Firouzeh igneous rocks, generated above the Sabzevar subduction zone. This subduction zone was active since Late Cretaceous time.

Recycling of palaeo-Pacific subducted oceanic crust related to a Fe–Cu–Au mineralization in the Xu-Huai region of North Anhui-Jiangsu, East China: Geochronological and geochemical constraints

ABSTRACTIn this study, Early Cretaceous skarn deposits and genesis of their host diorite/monzodiorite porphyry in the Xuzhou-Huaibei (Xu-Huai) region, northern Anhui-Jiangsu have been discussed by detailed geochemical work. In-situ zircon U–Pb dating of the diorites related to Fe–Cu–Au deposits shows that they were formed between 131.4 ± 1.5 Ma and 130.8 ± 1.8 Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry with similarity to that of arc-related igneous rocks. The diorite porphyry was probably derived from typical arc magmas related to continental margin subduction characterized by light rare earth elements (LREEs) enrichment and HFSE depletion. REEs compositions of apatite in the diorite porphyry indicate that the dioritic magma was produced from the metasomatized subcontinental mantle by slab-derived fluids. The magma was proven to be a high oxygen fugacity; thus, it was particularly conducive to the precipitation of Fe, Cu, Au and other ore-forming elements. The δ34S values of pyrite and chalcopyrite of Fe–Cu–Au ores range from −0.2‰ to 2.8‰, indicating that the sulphur in the ore was probably derived from deep-seated magmas. Integrated with geochronological and geochemical analyses, we suggest that the Early Cretaceous igneous suites associated with Fe–Cu–Au deposits in the Xu-Huai region are related to recycling subduction of Pacific oceanic crust.

Petrogenesis and geodynamic setting of Early Cretaceous felsic rocks in the Gan-Hang Belt, Southeast China: Constraints from geochronology and geochemistry of the tuffs and trachyandesitic rocks in Shengyuan volcanic Basin

The Late Mesozoic geology of the Gan-Hang Belt is characterized by extensive magmatism forming a belt of volcanic-intrusive complex. The geochronology, petrogenesis, and geodynamic setting of the Late Mesozoic magmatic rocks in the Gan-Hang Belt are still controversial. The Shengyuan volcanic Basin is located in the NW region of the belt and mainly contains crystal tuff, welded tuff, and trachyandesitic rocks. We integrate geochronological and geochemical data for these tuffs and trachyandesitic rocks, to explore the origin of these rocks and improve our understanding of the Late Mesozoic magmatic and tectonic evolution of the region. Zircon U-Pb dating shows that these samples were formed in the Early Cretaceous (135–137Ma). All the tuffs have a pronounced A2-type geochemical signature and their chemical compositions are controlled by crystal fractionation. All trachyandesitic rocks exhibit high K2O contents and were attributed to the shoshonite series; they are characterized by arc-like trace element distribution patterns, with significant enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depletion in high field-strength elements (HFSE). Moreover, all the tuffs and trachyandesitic rocks were characterized by negative whole rock εNd(t) and zircon εHf(t). An integrated interpretation of all these geochemical data leads to the conclusion that the Shengyuan trachyandesitic rocks were primarily derived from mantle materials and oceanic crust-derived melt. The Shengyuan tuffs were formed by the partial melting of the Proterozoic orthometamorphic and parametamorphic rocks. Our studies together with previous published data suggest that the Early Cretaceous A-type felsic rocks, with ages between 138Ma and 122Ma, occurred along the Gan-Hang Belt, indicating an important Late Mesozoic extensional event in the Belt. This event represents a back-arc tectonic setting due to the rollback of the Paleo-Pacific plate.

Petrogenesis of Late Cenozoic basaltic rocks from southern Vietnam

Major and trace element concentrations, and Sr–Nd–Hf–Pb isotopic compositions of Late Cenozoic (4.1 to 13.8Ma) basaltic rocks from southern Vietnam have been determined to understand the nature of their mantle source. The volcanic rocks are composed of tholeiite basalt, alkaline basanite, trachybasalt, basaltic trachyandesite, and trachyandesite. The alkaline rocks show light rare earth element (LREE) enrichment, with (La/Yb)N=10.3–29.8. The tholeiite basalts are distinguished by much lower values (8.8–9.5) of (La/Yb)N. On a primitive mantle-normalized trace element distribution diagram, they show oceanic island basalt (OIB)-like large-ion lithophile element enrichment without high field strength element depletion. However, some samples exhibit positive anomalies in K and Pb and negative anomalies in Sm, suggesting K-rich residual amphibole in the source. The samples contain Sr (87Sr/86Sr=0.703794–0.704672), Nd (ɛNd=+1.7-5.7), Hf (ɛHf=+4.0–10.9), and Pb (206Pb/204Pb=18.23–18.75; 207Pb/204Pb=15.53–15.59; 208Pb/204Pb=38.32–38.88) isotopes, plotting among OIBs, with depleted mid-ocean ridge basalt mantle-enriched mantle type 2 (DMM-EM2) characteristics. There are no discernible isotopic differences between tholeiite and the alkaline series, reflecting the same source. The Nd and Hf isotopic compositions are coupled, and plot along the mantle–crust array, ruling out the possibility of lithospheric mantle in the source. Plots of NiO against the Fo numbers of olivines from the basaltic rocks are within the range of Hainan and Hawaiian basalt olivines, implying that hybrid pyroxenite is present in the source. Also note that the estimated primary melt compositions fall within the experimental field defined by partial melting of silica-poor eclogite and peridotite. The effective melting pressure (Pf) and melting temperature (T) of the primary melts are Pf=29.6–32.8kbar and T=1470–1480°C. We suggest that Vietnamese basaltic rocks may be produced by the Hainan plume from recycled eclogitic oceanic crust.

Elemental and Sr–Nd isotopic geochemistry of the Uradzhongqi magmatic complex in western Inner Mongolia, China: A record of early Permian post-collisional magmatism

The magmatic complex in Uradzhongqi, Inner Mongolia, is located in the western segment of the northern margin of the North China Craton (NCC). The dominant components in the complex include syenogranite, monzogranite, granodiorite, diorite and gabbro. Mafic microgranular enclaves (MMEs) are common in syenogranite and granodiorite. Zircon U–Pb dating shows that the ages of these rocks range from 283 to 270Ma, suggesting an early Permian emplacement. The syenogranite and monzogranite are peraluminous I-type granites, exhibiting conspicuous negative Eu anomaly, enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE). The granodiorites, diorites and MMEs are metaluminous in composition, show high Al2O3, MgO and Fe2O3T contents and weak negative Eu anomaly, as well as LREE and LILE enrichment and HFSE depletion. The gabbros show weak positive Eu anomaly and slight REE differentiation. The Sr–Nd isotope compositions show that the source of mafic magma was depleted mantle (DM) with possible involvement of enriched mantle II (EM II), whereas the felsic magma was derived from the Archean lower crust. Petrographic observation and analytical results of mineralogy, geochronology, geochemistry and Sr–Nd isotopes indicate that the main petrogenesis of these magmatic rocks is the mixing of underplating mafic magma and felsic magma. Tectonically, the complex pluton was formed within a post-collisional regime, and the underplating in this area provides another piece of evidence for the vertical growth of the western segment of the northern margin of the NCC.

Carboniferous rifted arcs leading to an archipelago of multiple arcs in the Beishan–Tianshan orogenic collages (NW China)

The Beishan and East Tianshan Orogenic Collages in the southernmost Central Asian Orogenic Belt (CAOB) record the final stages of evolution of the Paleo-Asian Ocean. These collages and their constituent arcs have an important significance for resolving current controversies regarding their tectonic setting and age, consequent accretionary history of the southern CAOB, and the closure time of the Paleo-Asian Ocean. In this paper, we present our work on the southern Mazongshan arc and the northern Hongyanjing Basin in the Beishan Orogenic Collage (BOC), and our comparison with the Bogda arc and associated basins in the East Tianshan Orogenic Collage. Field relationships indicate that the Pochengshan fault defines the boundary between the arc and basin in the BOC. Volcanic rocks including basalts and rhyolites in the Mazongshan arc have bimodal calc-alkaline characteristics, an enrichment in large ion lithophile elements such as Rb, Ba, and Pb and depletion in high field-strength elements (e.g., Nb and Ta), which were probably developed in a subduction-related tectonic setting. We suggest that these bimodal calc-alkaline volcanic rocks formed in rifted arcs instead of post-orogenic rifts with mantle plume inputs. By making detailed geochemical comparisons between the Mazongshan arc and the Bogda arc to the west, we further propose that they are similar and both formed in arc rifts, and helped generate a Carboniferous archipelago of multiple arcs in the southern Paleo-Asian Ocean. These data and ideas enable us to postulate a new model for the tectonic evolution of the southern CAOB.

Recycling of Paleotethyan oceanic crust: Geochemical record from postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens

Plate tectonics can develop from oceanic subduction to continental collision, with disappearance of presubducted oceanic crust in continental subduction zones. This is typical for collisional orogens where Tethyan oceanic crust was consumed during continental convergence, with recycling of the Tethyan oceanic crust into the mantle. However, it is difficult to trace the geochemical signature of the previously subducted oceanic crust in collisional orogens. Postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens provide a geochemical record of crust-mantle interaction during Paleotethyan oceanic subduction preceding the Triassic continental collision. These mafic igneous rocks were emplaced in the Cretaceous after continental collision and exhibit oceanic-island basalt (OIB)−like trace-element distribution patterns, with enrichment of large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but no depletion of high field strength elements (HFSEs). They also show relatively depleted whole-rock Sr-Nd-Hf isotope compositions, with initial 87 Sr/ 86 Sr ratios of 0.7043−0.7050, e Nd ( t ) values of −1.8−4.5, and e Hf ( t ) values of 4.7−10.3. Such geochemical features indicate their origin from a juvenile mantle source that contained subducted oceanic crust−derived material. Thus, we suggest that the mantle source was generated by a melt-peridotite reaction at the slab-mantle interface in the Paleotethyan oceanic subduction channel during continental collision. The original peridotite would have been characterized by depletion of melt-mobile incompatible trace elements and radiogenic Sr-Nd-Hf isotopes, corresponding to the juvenile subarc lithospheric mantle. In contrast, the subducted crust−derived material would have been characterized by enrichment of LILEs and LREEs but no depletion of HFSEs, corresponding to felsic melts that were derived from partial melting of the subducting Paleotethyan oceanic basalt and sediment outside the rutile stability field. As such, the melt-peridotite reaction would generate pyroxenite-rich ultramafic metasomatites in the lower part of the mantle wedge, with relative enrichment of LILEs and LREEs but no depletion of HFSEs. In this model, partial melting of the metasomatites gave rise to the postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens. Therefore, the postcollisional mafic igneous rocks record the recycling of Paleotethyan oceanic crust in the collisional orogen.

Petrogenesis of the ∼500 Ma Fushui mafic intrusion and Early Paleozoic tectonic evolution of the Northern Qinling Belt, Central China

The Fushui mafic intrusion in the Qinling orogenic belt (QOB) is composed of meta-gabbro, meta-gabbro-diorite, diorite, and syenite. Most of these rocks are metamorphosed under the upper greenschist facies to lower amphibolite facies metamorphism. Zircon separates from eight samples have LA-ICP-MS U-Pb ages of 497–501Ma which are taken to be the emplacement age of magmas that formed the Fushui intrusion. Most of the zircon grains exhibit negative εHf values, correspond to TDM2 model ages of late Paleoproterozoic-early Mesoproterozoic or Neoproterozoic and suggest that the mafic rocks were most probably derived from mafic melts produced by partial melting of a previously metasomatized lithospheric mantle.The intrusion is not extensively contaminated by crustal materials and most chemical compositions of rocks are not modified during the greenschist to amphibolite-facies metamorhism. Rocks from the intrusion have primitive mantle-normalized trace element patterns with significant enrichment in light-REE and large ion lithophile elements (LILE) and depletion in high field-strength elements (HFSE). On the basis of the trace element contents, the Fushui intrusion was derived from parental magmas generated by <10% partial melting of both phlogopite-lherzolite and garnet-lherzolite mantle sources. These sources are best interpreted to be in a subduction-related arc environment and have been modified by fluids released from a subducting slab. The formation of the Fushui intrusion was related to the subduction of the Paleotethyan Shangdan oceanic lithosphere at ∼500Ma.

Shoshonite-latite series of the Eastern Transbaikalia: 40Ar/39Ar age, geochemistry, and Sr-Nd isotope composition of rocks from the Akatui volcano-plutonic association of the Aleksandrovskii Zavod depression

The paper presents new data on age, geochemistry, and Sr and Nd isotope composition of rocks from the Akatui massif and comagmatic rocks from the lower unit of the Kailas Formation (Akatui volcano-plutonic association), localized within the Aleksandrovskii Zavod depression. The amphibole 40Ar/39Ar age date the monzogabbro of the early phase of the Akatui massif at 154.8 ± 4.4 Ma; the monzonite of the main phase yields a 40Ar/39Ar age of 160.7 ± 3.9 Ma, and the shoshonite basalt of the lower unit of the Kailas Formation yields a 40Ar/39Ar age of 161.5 ± 1.7 Ma. The leading petrogenetic mechanism for the Akatui volcano-plutonic association is crystal fractional differentiation of melts with minor crustal contamination, which can be suggested from the mineralogical and petrographic features and geochemical and isotope characteristics of rocks. The geochemical data for the Akatui volcano-plutonic association show LILE, LREE, U, Th, and Pb enrichment with a characteristic depletion in high-field strength elements (HFSE), such as Nb and Ti. They are also depleted in P. Sr-Nd isotope data (87Sr/86Sr(160 Ma) = 0.70642-0.70688 and £Nd(160 Ma) = −0.6 to −2.2) suggest an EMII-type mantle source and could also indicate a negligible degree of crustal contamination in the evolved melts.

A record of post-collisional transition: evidence from geochronology and geochemistry of Palaeozoic volcanic rocks in the eastern Junggar, Central Asia

ABSTRACTAbundant clues about the post-collisional transition in the southwestern margin of the Central Asian Orogenic Belt (CAOB) have been recorded by the late Palaeozoic volcanic rocks in the eastern Junggar region. These volcanic rocks provide a better window for understanding the magmatic and tectonic transition processes. This paper reports detailed petrology, whole-rock and Sr–Nd–Pb isotope geochemistry, and in situ zircon U–Pb dating analyses of these volcanic rocks from this region. Systematic geochemical and geochronological studies show that three stages of magmatism were recorded in this area, including ca. 407 Ma and ca. 340 Ma medium-to-high-K calc-alkaline series and ca. 320 Ma alkaline and tholeiite series. The ca. 407 Ma volcanic rocks have calc-alkaline compositions, strong enrichment in large-ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), negative Nb anomalies, and light REE (rare earth element)-enriched patterns. The ca. 340 Ma volcanic rocks, which have similar calc-alkaline compositions and arc geochemical affinities but higher K content, have slightly lower Th/Yb and Ba/Nb values than typical island arc or continental margin volcanic suites. The compositions of the ca. 320 Ma volcanic rocks display a bimodal distribution; the ca. 320 Ma basaltic rocks are chemically similar to oceanic island basalt (OIB) but with slight depletion in Nb and Ta, wheres the acidic rocks show the features of A-type granite. The chemical compositions of these volcanic rocks indicate that the ca. 407 Ma magmatism was subduction related, while the ca. 340 Ma and ca. 320 Ma volcanic rocks were the product of post-collisional magmatism. This transition from ca. 407 Ma to ca. 320 Ma records slab subduction, slab break-off, and slab rollback and provides a plausible explanation for the complex spatial and temporal geochemical evolution of magmatism in eastern Junggar since the Devonian.

U–Pb zircon geochronology and geochemical constraints on the age and origin of late Neoarchean leucosomes in migmatites from the Maevatanana area, Madagascar

Migmatites represent the dominant lithological facies in the Maevatanana area of Madagascar. The migmatites are composite rocks with hybrid metamorphic and magmatic features, comprising 60% melanosome bands that are interlayered and/or tectonically interleaved with 40% of centimeter-to decimeter-scale quartz–feldspar leucosome veins. The leucosome and granite veins are bordered by a series of gently dipping shear zones in the migmatites. U–Pb zircon dating shows that the leucosome veins formed at 2548.3 ± 5.3 Ma and are coeval with intrusion of the granite veins at 2552.2 ± 6.1 Ma. Furthermore, the leucosome and granite veins have uniform chondrite-normalized rare earth element (REE) patterns which are characterized by light REE enrichment relative to heavy REE, and negative or slightly positive Eu anomalies (δEu = 0.61–1.13). Primitive-mantle-normalized trace element patterns of the leucosome and granite veins are enriched in large-ion lithophile elements (Ba and K) and show pronounced depletions in high-field strength elements (Th, U, Ta, Nb, Zr, Hf, and Ti), suggesting that the leucosome and granite veins in the migmatites are syntectonic and synigneous intrusives. Primitive-mantle-normalized trace element patterns of the leucosome veins and melanosome bands in the migmatites are similar and suggest that the granite veins in the migmatites were generated by partial melting of medium-to high-K metabasalt, whereas the melanosome bands originated through metamorphism of calc-alkaline basalts. As such, the leucosome veins were likely generated by partial melting of melanosome bands in the migmatites. Calculated zircon saturation temperatures (TZr) of the leucosome veins are 599–685 °C, which represents the magma crystallization temperatures, and the pressures of magma formation range from 1.2 to 3.2 GPa. These P–T estimates for magma formation and crystallization are consistent with those of 605–658 °C and 1.22–1.43 GPa for metamorphism of the melanosome bands in the migmatites, suggesting that the leucosome veins in the migmatites were formed during peak metamorphism of the melanosome bands.

Geochemical characteristics of the Shujiadian Cu deposit related intrusion in Tongling: Petrogenesis and implications for the formation of porphyry Cu systems in the Middle–Lower Yangtze River Valley metallogenic belt, eastern China

Porphyry Cu deposits can form in intracontinental or post-collision settings; however, both the genesis of fertile magmas and the mechanism of metal enrichment remain controversial. The Shujiadian porphyry Cu deposit is located in the Tongling area of the Middle–Lower Yangtze River Valley metallogenic belt. It is hosted by the Shujiadian complex, which mainly consists of quartz diorite porphyry (143.7±1.7Ma) and pyroxene diorite (139.8±1.6Ma). They both belong to the calc-alkaline series, with enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field-strength elements (HFSE) and heavy rare earth elements (HREE), and slightly negative Eu anomalies. Both quartz diorite porphyry and pyroxene diorite have geochemical affinities with adakite, and their low MgO (1.5–3.7wt%), and Ni (3.7–6.9ppm), Cr (2.0–44ppm), and Th/Ce contents (0.06–0.11) indicate that the intrusive rocks have some characteristics of adakite-like rocks derived from thickened lower crust and melts from metabasaltic rocks and eclogites. Plagioclases from the quartz diorite porphyry are andesine (An value=31.8–40.5) and from the pyroxene diorite are felsic albite and oligoclase with large-scale zones and variable An value (An value=8.9–18.3), Fe and Sr contents, which indicate that mixing of mafic and felsic magma may have occurred in the shallow magma chamber. Compared to the barren quartz diorite porphyry, relatively lower SiO2 contents (49.5–55.2wt.%), higher εNd(t) values (−7.4 to −6.9), εHf(t) values (−11.0 to −9.1) compositions, Ti-in-zircon temperatures (714–785°C), and variations of HREE contents of the mineralization-related pyroxene diorite suggest mixing with high-temperature mafic magma. Calculated Ce4+/Ce3+ values of pyroxene diorite plot between the Ni–NiO buffer (NNO) and magnetite–hematite buffer (MH), and barren quartz diorite porphyry samples plot below the Ni–NiO buffer (NNO). Geochemical features of relatively enriched LILEs, depleted HFSEs, and markedly negative Nb–Ta anomalies, imply that the mafic magma was generated by partial melting of enriched lithospheric mantle, which had been metasomatized by slab-derived fluids from Neoproterozoic-subducted oceanic lithosphere; as a result, the parental magma was rich in H2O, S, and metals (e.g. Cu), with high fO2.

Metallogenesis and ore controls of Cenozoic porphyry Mo deposits in the Gangdese belt of southern Tibet

The Gangdese is a newly explored porphyry copper ore belt in China. Except for the Cu (±Mo±Au) porphyry deposits there are some Mo (±Cu±W) porphyry and skarn deposits in this belt. Two pulses of molybdenite mineralization are recognized in the central Lhasa subterrane with ancient continent crust, including the Paleocene–Eocene (65–52Ma) porphyry Mo and skarn Mo–W deposits formed during the rollback of Neo-Tethyan oceanic slab in collisional setting, and Miocene (21–15Ma) porphyry Mo–Cu deposits generated in the postcollisional setting. The Gangdese copper deposits also occur during these two periods but are distributed in the southern Lhasa subterrane dominated by juvenile crust through mantle-derived magmatism resulting from Neo-Tethyan ocean lithosphere subduction. The intrusions related to molybdenite mineralization have relatively lower bulk-rock εNd(t) and higher (87Sr/86Sr)i values, lower zircon εHf(t) values and older Hf model ages, lower molybdenite Re contents, and more radiogenic Pb isotopes than the coeval copper related porphyries in the Gangdese belt. In addition, the Miocene Mo–Cu related intrusions generally have lower zircon Ce4+/Ce3+ ratios than the Miocene Cu (±Mo) related intrusions. These data indicate that more contributions from mature continental crustal materials play crucial role in the generation of Mo than Cu porphyry mineralization in the Gangdese belt.The Eocene Mo related intrusions are characterized by high SiO2 (68–76%) and K2O (5.1–5.9%) concentrations, enrichment of large-ion lithophile elements (LILE), depletion of high field strength elements (HFSE), and low Sr/Y (7.5–23.8) ratios. Their Sr–Nd isotope compositions (εNd(t)=−3.4 to −4.6; 87Sr/86Sr(i)=0.70622–0.70679) indicate that they are associated with partial melting of mantle wedge mixing with significant amounts (55%–65%) of middle-upper crustal materials. The Miocene Mo–Cu related intrusions have relatively high SiO2 (64%–73%) and K2O (1.9–4.8%) contents and are characterized by enrichment of LILE, depletion of HFSE, and high Sr/Y ratios (55–189). Their Sr–Nd isotope compositions (εNd(t)=−4.6 to −3.4; 87Sr/86Sr(i)=0.70622–0.70679) are indicative of derivation from remelting of previously subduction modified Tibetan lithosphere with significant amounts (50%–70%) of middle-upper crustal materials. For the Gangdese Mo (±Cu) porphyry mineralization, Mo metal was suggested to be derived from the middle–upper crustal materials, particularly the Carbonaceous–Permian marine sedimentary, whereas Cu metal to be sourced from the Gangdese Miocene Cu fertile magmas. The exploration targeting of the Gangdese molybdenite deposits should be focused on southern part of the central Lhasa subterrane where the Carbonaceous–Permian strata occur. The occurrence of Mo–Cu porphyry deposits could provide clue for targeting Cu–Mo porphyry deposits in the Gangdese belt. The Gangdese molybdenite deposits share many similarities with the East Qinling molybdenite deposits, such as type of ore deposits, geodynamic setting of their formation, variable Mo grades and high F contents.

The ∼860 Ma mafic dikes and granitoids from the northern margin of the Yangtze Block, China: A record of oceanic subduction in the early Neoproterozoic

There are voluminous Neoproterozoic arc-related volcano-sedimentary sequences and small intrusions on the northern margin of the Yangtze Block, South China. The understanding the origin of the Sanligang granitoid intrusion and the spatially associated mafic dikes in the region is crucial for unraveling the tectonic evolution and continental crust growth processes in the Yangtze Block. Zircon U–Pb dating suggests that the mafic dikes (ca. 871Ma) and granitoids (ca. 860Ma) are contemporaneous. The mafic dikes have low SiO2 (45.37–46.55wt.%), K2O (0.32–0.82wt.%) and Na2O (2.01–2.85wt.%), and are characterized by enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), suggesting that their mantle source was modified by subducted materials. The Sanligang granitoids have intermediate to high SiO2 (60.35–71.38wt.%), intermediate K2O (1.38–3.67wt.%) and Na2O (3.97–5.33wt.%), and high MgO (1.03–3.16wt.%). They show LREE-enriched REE patterns (La/YbN=7.2–12.3) with no or minor negative Eu anomalies. Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dikes and granitoids share similar zircon ɛHf(t) values (+10.5 to +12.9, +7.9 to +11.7, respectively), whole-rock initial 87Sr/86Sr ratios (0.7051–0.7057, 0.7033–0.7041, respectively) and ɛNd(t) values (+4.0 to +7.1, +3.4 to +4.9, respectively), suggesting that the granitoids were generated by partial melting of juvenile basaltic crust. High Mg# values (49–58) in the granitoids may have resulted from assimilation of residual mafic minerals in their source region. Based on its arc-related geochemical affinity and contemporaneous arc-related magmatism, the Sanligang pluton is proposed to be generated in a Neoproterozoic arc setting during crustal growth and reworking. The early Neoproterozoic assemblage from the Sangligang–Sanyang fault belt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block.

Petrogenesis of Late Devonian–Early Carboniferous volcanic rocks in northern Tibet: New constraints on the Paleozoic tectonic evolution of the Tethyan Ocean

Paleozoic arc-type volcanic rocks in northern Tibet, provide important information on the Paleozoic tectonic evolution of the Longmu Co–Shuanghu Tethyan Ocean (LSTO). In this study, we obtained zircon U–Pb and Hf isotopic data, and major and trace element data for volcanic rocks from the Riwanchaka and Laxiongco areas in northern Tibet. LA-ICP-MS U–Pb analyses of zircons show that volcanic rocks in the Riwanchaka and Laxiongco areas formed at 374–372 and 356–353 Ma, respectively. These calc-alkaline volcanic rocks consist of basalt, andesite, dacite, and rhyolite. The basalt and andesite in the Riwanchaka area exhibit low TiO2 and high MgO and Na2O concentrations, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). The basalt probably originated from partial melting of a lithospheric mantle source, metasomatized by subduction-related fluids, whereas the andesite was probably derived from hydrous basalts after fractional crystallization. The Riwanchaka and Laxiongco rhyolites are characterized by high Na2O/K2O ratios and low Al2O3 concentrations, are weakly metaluminous to peraluminous (A/CNK=0.97–1.11), and exhibit typical arc-like geochemical signatures (e.g., enrichment in LILEs and strong depletion in HFSEs and Sr). In addition, they show high zircon εHf(t) values (+7.61 to +4.45). The geochemical characteristics and zircon εHf(t) compositions of the rhyolites indicate that they were derived from partial melting of juvenile basaltic lower crust. These results, combined with previous data, indicate that the LSTO lithosphere was subducted northward beneath the North Qiangtang subterrane (NQT) during the Late Devonian–Early Carboniferous. We suggest that the NQT represents a slice of an active continental margin developed on the southern margin of the Eurasian continent during the Late Devonian–Early Carboniferous.

Boninitic geochemical characteristics of high-Mg mafic dykes from the Singhbhum Granitoid Complex, Eastern India

The high-Mg mafic dykes from the Singhbhum Granitoid Complex in East India have geochemical characteristics [e.g., enrichment of the large ion lithophile elements and light rare earth elements (LREEs) relative to high field strength elements (HFSEs): high-MgO (>8 %), high-SiO2 (≥52 %), low-TiO2 (≤0.5 %), and high CaO/Al2O3 (≥0.58)] similar to those found in boninitic/noritic rocks. Their high percentage of orthopyroxene as a mafic mineral and of plagioclase as a felsic mineral, and normative hypersthene content greater than diopside content are also indications of their boninitic/noritic affinity. On a triangular diagram of MgO-CaO-Al2O3 and on binary diagrams of Ti/V vs Ti/Sc and TiO2 vs Zr, these samples show geochemical similarities with Phanerozoic boninites and Paleoproterozoic high-Mg norites. On major and trace element variation diagrams, these dykes show a normal crystallization trend and their Nb/La (<0.5) and Nb/Ce (<0.21) values lower than average bulk crust (0.69 and 0.33, respectively) suggest no crustal contamination. Their low values of Rb/Sr (0.11–0.41) and Rb/Ba (0.10–0.27) also suggest little or no effect of post magmatic processes. Their TiO2 (0.27–0.50), Al2O3 /TiO2 (19.30–42.48), CaO/TiO2 (12.96–32.52), and Ti/V (12–18) values indicate derivation from a depleted mantle source under oxidizing conditions such as a mantle wedge. Ni vs Zr modeling shows that the studied high-Mg dykes were generated by 25–30 % melting of a refractory mantle source. Enrichment of Rb, Th, U, Pb, Sr, and LREEs, and depletion of HFSEs—especially Nb, P, Ti, Zr—on primitive mantle—and chondrite-normalized spider diagrams, respectively, are clear signals that the slab-derived component played an important role in the formation of melts for these rocks in a supra-subduction zone setting.

Geochronology and geochemistry of the Nanfen iron deposit in the Anshan-Benxi area, North China Craton: Implications for ∼2.55 Ga crustal growth and the genesis of high-grade iron ores

The Nanfen iron deposit, situated in the Anshan-Benxi area of the North China Craton (NCC), hosts scattered banded iron formations (BIFs) and BIF-hosted high-grade iron orebodies (defined by FeOT>80wt.%). The associated roof and bottom rocks, which occur in parallel to orebodies, are mica quartz schist and chlorite hornblende schist (locally interlayers), respectively. The PAAS-normalized rare earth element (REE) patterns indicate that quartz–magnetite banded iron formations (BIFs) are enriched in Heavy REEs with positive La, Eu and Y anomalies, indicating that the Nanfen BIF precipitated from a mixture of ambient seawater and submarine high-T hydrothermal fluids (∼0.1%). In addition, the BIF exhibits low concentrations of Al2O3 and TiO2 with depletion in HFSEs (high field strength elements, e.g., Nb, Ta, Zr, Hf and Ti), suggesting little contamination with continental detritus. Excepting lower Eu/Eu* values, the high-grade iron ores display similar geochemical features to the BIF, likely implying that they have a same iron source but did not form synchronously. Electron microprobe analyses (EMPA) and in situ LA-ICP-MS analyses reveal that in relative to the magnetite grains in high-grade ores, the magnetite grains in the BIF contain higher MnO, MgO and Zn concentrations, and lower Al2O3 and REE abundances, implying different formation conditions. Thus, we argue that high-grade ores are likely to be alteration products of primary BIFs. Chlorite hornblende schist (meta-basalt) samples exhibit low SiO2 contents (43.78–50.30wt.%) with flat chondrite-normalized REE and N-MORB-normalized trace element patterns, which are similar to those of BABB (back-arc basin basalt). The zircon U–Pb dating results indicate that magmatic and metamorphic zircons from chlorite hornblende schist formed at approximately 2552Ma and 2482Ma, respectively. The former volcanic age could constrain the deposition age of the Nanfen BIF, and the latter reflects a subsequent metamorphic event, which is also reflected by the ∼2480Ma hydrothermal zircons selected from the BIF. Moreover, most zircons from chlorite hornblende schist have positive ɛHf(t) values (0.22–7.96) close to the depleted mantle evolutionary curve, implying that the source magma were derived from a depleted mantle with some contamination from ancient crust. Hence, we propose that the Nanfen BIF is a Neoarchean Algoma-type BIF, and its deposition was most likely benefited due to crustal growth.

The Lamandau IOCG deposit, southwestern Kalimantan Island, Indonesia: Evidence for its formation from geochronology, mineralogy, and petrogenesis of igneous host rocks

The Lamandau region of Kalimantan Island, Indonesia is located in Sandaland, in the southern part of the Kuching tectonic belt. A series of Cenozoic epithermal gold deposits and Fe–Cu–Au deposits are located in the Kuching belt. The Lamandau Fe–Cu–Au deposit is hosted by diorite porphyry. In-situ zircon U–Pb dating of the diorite porphyry shows that it formed between 82.1±1.7Ma and 78.7±2.3Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry and related basalt is similar to that of arc-related igneous rocks. The diorite porphyry and basalt were probably derived from typical arc magmas related to continental margin subduction and thus are characterized by light rare earth element (REE) enrichment and HFSE depletion. The sub-chondritic Nb/Ta ratios for the basalt in the Lamandau region indicate that the subducted Pacific slab began partial melting at depths where amphibole was the major residual phase, with some residual rutile. The basalt was derived from a depleted mantle source. The composition of apatite and zircon in the diorite porphyry indicates that the dioritic magma was produced from the subcontinental mantle after it was metasomatized by slab-derived fluids. The magma had a high oxygen fugacity, thus and therefore it was particularly conducive to the precipitation of Cu, Au and other ore-forming elements. The composition of magnetite indicates that it was of volcanic origin. The magnetite has a low REE content, and a high Cu–Au content. The deposit may be classified as an IOCG mineral system. In summary, the ore-related diorite porphyry in the Lamandau region might have formed in an extensional environment during rollback of the subducting western Pacific plate. The convergent velocity between the Philippine Sea and Eurasian plates was at a minimum during the rollback, so that the margin of East Asian began to undergo rifting with associated magmatism.

Geochronology and geochemistry of Grenville-aged (1063 ± 16 Ma) metabasalts in the Shennongjia district, Yangtze block: implications for tectonic evolution of the South China Craton

To better understand the evolution of the South China Craton (SCC), we have determined the geochronological and geochemical compositions of newly recognized Grenville-aged metabasalts in the Shennongjia region of the northern Yangtze block. LA-ICP-MS U-Pb dating of zircons indicates that the metabasalts formed at 1063 ± 16 Ma. The rocks are calc-alkaline, are characterized by SiO2 contents (50.50 to 55.62 wt.%), and have moderate-to-high MgO contents (7.25–9.60 wt.%). They display light rare earth element enrichment ((La/Yb)N = 7.0–9.8) with slightly negative Eu anomalies (Eu/Eu* = 0.82–0.90) and have pronounced depletion in high-field strength elements as well as positive Pb anomalies in the primitive mantle-normalized trace element pattern. They possess high initial Sr isotopic ratios of 0.7092–0.7107, large negative εNd(t) values of −12.1 to −11.0, and a relatively narrow range of initial Pb isotope ratios (206Pb/204Pb = 16.503–17.019, 207Pb/204Pb = 15.259–15.452, and 208Pb/204Pb = 36.169–36.994). These isotopic characteristics are typical of basalts derived from an EM2 source region and suggest a subcontinental lithospheric mantle source that was metasomatized by subducted components (fluids and melts). Integrating our new data with documented igneous and metamorphic events during late Mesoproterozoic to early Neoproterozoic time in the region and western segments of the Yangtze block, we suggest that the Shennongjia area might be a microcontinent that was independent of the continental nucleus of the Yangtze block and was accreted to the Yangtze block at the end of Mesoproterozoic time. This study thus argues against the traditional view that the SCC was formed simply by Yangtze–Cathaysia collision and supports a hypothesis in which the Yangtze block was a collage of microcontinents accreted during the Grenvillian period accompanying the assembly of the Rodinia supercontinent.