Low TiO2 Contents Research Articles

Geodynamic setting, structure, and composition of continuous trachybasalt-trachyandesite-rhyolite series in the north of Altai-Sayan area: the role of crust-mantle interaction in continental magma formation

The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO2 contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO2 (<1.7 wt.%) and Fe2O3tot and higher contents of Al2O3 in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.

Petrogenesis and evolution of the Nuweibi rare-metal granite, Central Eastern Desert, Egypt

The Nuweibi rare-metal granite in the Central Eastern Desert of Egypt is highly evolved fine- to medium-grained leucogranite affected by pervasive albitization and greisenization. The intrusion holds an important tin–tantalum resource in the Egyptian Eastern Desert. Columbite–tantalite and cassiterite disseminations occur within the granite body, while the quartz ± feldspar veins cutting across the Nuweibi granite host only cassiterite disseminations. Microscopically, quartz and alkali-feldspar are the essential mineral constituents of Nuweibi granite, with minor mica (muscovite + rare biotite), while cassiterite, columbite–tantalite, zircon, allanite, beryl, tourmaline, titanite, and fluorite are accessories. Whole-rock geochemistry and micoanalytical data together with laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) dating of zircon and columbite have been used to constrain the evolution of the granite intrusion and associated mineralization. The Nuweibi granite is weakly peraluminous with extremely low MgO, CaO, TiO2, P2O5, Ba, and Sr contents and elevated Sn, Ta, Nb, and Rb contents. The REE patterns exhibit distinct tetrad effects, as well as negative Eu and Y anomalies. Also, the bulk rock Zr/Hf ratios are consistently < 10. The Nd isotopic system is disturbed and eNd values suggest a juvenile mantle and/or Neoproterozoic crustal source. The U–Pb system in zircon is disturbed and leaked continuously, while the U–Pb age of columbite is ~ 620 Ma. The geochemical and isotopic systematics of the Nuweibi intrusion reflect very advanced degree of fractionation combined with late magmatic fluid overprint which redistributed Sn and other mobile elements, while Ta still characterizes the igneous system.

Evidence for magma–evaporite interactions during the emplacement of the Central Atlantic Magmatic Province (CAMP) in Brazil

Mafic sills of the Central Atlantic Magmatic Province (CAMP) in Brazil are temporally linked to the end-Triassic extinction (ETE), one of the largest mass extinctions of the Phanerozoic. The sills were emplaced into the volatile-rich sedimentary rocks of the Amazonas and Solimões basins in northern Brazil. Here we present a new geochemical study of 26 dolerite samples from 6 deep boreholes in the Brazilian basins, including whole-rock major and trace elements, whole-rock Sr–Nd isotopes and detailed biotite mineral chemistry. The bulk of the dolerites are characterized by phenocrysts of clinopyroxene and plagioclase in subophitic to intergranular textures, Fe–Ti oxides, and rare olivine and orthopyroxene. A different mineralogical assemblage (microphenocrysts of alkali-feldspar, quartz, biotite and apatite) is found in small independent domains, localized within the framework of coarser plagioclase and clinopyroxene laths. These fine-grained evolved domains crystallized in late-stage, evolved melt pockets in the interstitial spaces between earlier crystallized coarser grained crystals. The majority of the studied dolerites are generally evolved tholeiitic basalts and basaltic andesites with low TiO2 concentrations (<2.0 wt.%). Four samples have high TiO2 concentrations (>2.0 wt.%), and are found in the eastern part of the Amazonas Basin. Whole-rock major and trace element and Sr–Nd isotope geochemistry of both low- and high-Ti sills is similar to that of previously published CAMP rocks from the two magma types. Low-Ti sills show enriched isotopic signatures (143Nd/144Nd201Ma 0.51215 to 0.51244; 87Sr/86Sr201Ma 0.70568 to 0.70756), coupled with crustal-like characteristics in the incompatible element patters (e.g. depletion in Nb and Ta). Unaltered high-Ti samples show more depleted isotopic signatures (143Nd/144Nd201Ma 0.51260 to 0.51262; 87Sr/86Sr201Ma 0.70363 to 0.70398). Low-Ti dolerites from both the Amazonas and Solimões basins contain biotite with extremely high Cl concentrations (up to 4.7 wt.%). We show that there is a strong correlation between host-rock lithology and Cl concentrations in biotite from the dolerites, and interpret this to reflect large-scale crustal contamination of the low-Ti magmas by halite-rich evaporites. Our findings support the hypothesis that sill-evaporite interactions increased volatile release during the emplacement of CAMP. This strengthens the case for active involvement of this LIP in the end-Triassic crisis, and suggests that the sub-volcanic part of a LIP can be of major importance in driving climate change and mass extinctions.

Petrogenesis of Cenozoic high–Sr/Y shoshonites and associated mafic microgranular enclaves in an intracontinental setting: Implications for porphyry Cu-Au mineralization in western Yunnan, China

Cenozoic high–Sr/Y shoshonites in western Yunnan, China, are spatially and temporally associated with porphyry deposits and mafic volcanic rocks that formed at a post–collisional setting. However, the petrogenesis of these rocks and the origins of the associated mineralization remain unclear. Here we present new geochemical data for the high–Sr/Y shoshonites and associated mafic microgranular enclaves (MMEs) in the Jinshajiang–Ailaoshan mineralization belt, which formed as a result of India–Asia collision during the Cenozoic. Both fertile and barren shoshonitic porphyries were emplaced during the Eocene–Oligocene (32–37 Ma) and are characterized by enrichment in large–ion lithophile elements (LILE), depletion in high field strength elements (HFSE), high K2O (4–6 wt%), Sr contents (674–1370 ppm), and low Y contents (7–18 ppm). The barren Zhanhe, Yongsheng, Songgui, and Jianchuan, and fertile Machangqing (Cu–Au–Mo mineralization) porphyry intrusions have variable zircon εHf(t) values of −6 to +9 and old TDM2 ages of 1433 to 900 Ma. However, the barren Shigu porphyry intrusion yields relatively positive zircon εHf(t) values (−4 to +8) and younger TDM2 ages (903–631 Ma). These features indicate that the Shigu intrusion was derived from late Permian juvenile lower crust, and the other high Sr/Y shoshonites were derived mainly from Neoproterozoic juvenile lower crust. Fertile and barren porphyries have variable whole–rock εNd(t) values (−5 to −0.7) and (87Sr/86Sr) i ratios (0.70595–0.70788) that are within the range of coeval mafic magmas in western Yunnan. The MMEs comprise mafic enclaves and andesitic enclaves that formed during the Eocene (34–35 Ma) and have spheroidal shapes, igneous textures and contain acicular apatites. This indicates that the MMEs are globules of coeval mafic magma that was injected into and mixed with the host shoshonitic magma. The enclaves show low TiO2 contents (0.7–0.8 wt%), high MgO (6–9 wt%), Cr (184–450 ppm) and Ni contents (98–157 ppm), positive LILE anomalies, marked negative HFSE anomalies, a narrow range of εNd(t) values (−1.2 to −0.6), and (87Sr/86Sr) i values of 0.70556–0.70635. The andesitic enclaves show adakitic geochemical affinities and have similar zircon εHf(t) values (−2.7 to +2.5) to the host porphyries, whereas, the mafic enclaves have relatively negative zircon εHf(t) values (−8.7 to +1.5), similar to enriched mantle. These evidences suggest the high−Sr/Y shoshonites were produced by partial melting of juvenile lower crust mixed with ultrapotassic or potassic mafic magmas as represented by the andesitic enclaves. The mafic enclave melts were derived from enriched mantle metasomatized by slab–derived fluids, which elevated the Mg#, Cr, Ni contents of the host porphyries at Machangqing, Zhanhe and Yongsheng.Zircon saturation temperatures (TZr) of the high–Sr/Y shoshonitic rocks range from 735 to 777 °C and indicate the shoshonites were derived mainly from water–fluxed melting of lower crust. The Manchangqing fertile intrusion had a high oxidation state (zircon Ce4+/Ce3+ = 38–180, average = 91) and was derived by mixing between juvenile lower–crustal melts and voluminous coeval lamprophyres. In contrast, the barren Jianchuan, Songgui, Shigu, Yongsheng and Zhanhe high–Sr/Y shoshonites formed by partial melting of juvenile lower crust and some mixing with mafic lavas, and are characterized by low oxygen fugacity (zircon Ce4+/Ce3+ = 0.5–77, average = 20). As such, we suggest the mafic magmas triggered water–fluxed, moderate–degree partial melting of the lower crust and caused amphibole breakdown during melting. This process could not provide enough water for the formation of porphyry Cu-Au deposits in western Yunnan, and therefore formed low–fO2, relatively low–H2O, and barren high–Sr/Y magmas. However, ultrapotassic magmas (e.g., lamprophyres) with high H2O are able to trigger intense water–fluxed, partial melting of metal–fertilized lower crust, which generated high–fO2, relatively high–H2O, fertile, high–Sr/Y shoshonitic magmas in this intracontinental setting.

Variably evolved gabbroic intrusions within the Xigaze ophiolite (Tibet): new insights into the origin of ophiolite diversity

Ophiolites worldwide show striking diversities in their rock assemblage and structure (i.e., ophiolite diversity), raising a question whether ophiolites are originally similar before intense tectonic dismemberment. Comparison between ophiolites and oceanic lithospheres at modern mid-ocean ridges may provide key constraints on the origin of ophiolite diversity, because oceanic lithospheric structures are inherently controlled by spreading rates. Here, we present a case study of the Xigaze ophiolite in southern Tibet focusing on its gabbroic intrusions outcropping in three localities, i.e., Dazhuqu, Baigang and Jiding. Compared to the Jiding sequence, the Dazhuqu and Baigang gabbroic rocks are less evolved, characterized by higher Cr2O3 contents but lower contents of TiO2 and rare earth element in both clinopyroxene and bulk compositions. It is evident, hence, that the Xigaze ophiolite is characterized by variably evolved and discontinuously distributed gabbroic intrusions, rather by a continuous lower oceanic crust between the mantle and sheeted dike complex as the Penrose-type ophiolites. Our study, along with the identification in previous studies of oceanic detachment faults within the Xigaze ophiolite, demonstrates that the Xigaze ophiolite shows close similarities to oceanic lithospheres at modern slow- and ultraslow-spreading ridges. Hence, the significant structural distinctions between the Xigaze ophiolite and the Penrose-type ophiolites (e.g., the Oman ophiolite) may be inherently associated with different spreading rates of paleo-ridges. Considering the limited scale of the Xigaze gabbroic rocks, here we suggest the Xigaze ophiolite as a typical representative of fossil ultraslow-spreading ridges.

Iron ore deposits associated with Hormuz evaporitic series in Hormuz and Pohl salt diapirs, Hormuzgan province, southern Iran

Three types of iron ore, including banded ore, hydrothermal-metasomatic ore and earthy hematite ochre occur in Hormuz and Pohl salt diapirs. Primary banded ores of upper Neoproterozoic age contain alternating iron-rich and silica-rich bands, which characterized by the presence of folded hematite-rich bands and glacial diamictite fragments of quartz, dropstone and other clastics. Hematite as specularite and oligist is the major mineral in the iron-rich bands with minor goethite, pyrite, magnetite and rare siderite. Textural relationships between the ore and gangue minerals indicate that these ores are Rapitan-type sedimentary banded iron deposits. This is supported by the geochemical data of iron ores, such as low concentration of Al2O3, TiO2, Ba, V, Cr, Co, Ni, Cu and other trace elements, plus high value of MnO, Fe/V and Fe/Ti ratios, and REE patterns. Subsequent to the formation of iron ore deposits, an immense thickness of evaporitic rocks was formed in the oceanic marginal basins. Negative δ13C and δ18O values of the cap carbonates and strong positive δ34S values of the underlying sulfates are similar to the isotopic patterns of Ediacaran Rapitan-type iron deposits. Due to salt diapirism in the upper Mesozoic and Cenozoic eras, secondary iron deposits were formed by the activation of hot hydrothermal fluids during salt rising. High contents of some indicative elements such as Sr, Zr, U, Th, Pb, Zn, Sn, W and Mo in the metasomatic iron ores showed that they are the products of iron ore remobilization–redeposition in a hydrothermal system. The nearness of these ores to the fault zones is consistent with the scheme of migration and redeposition of iron compounds as metasomatic deposits under oxidizing conditions. Finally, earthy ochre was formed following the weathering and erosion of pre-existent iron ores.

Chromian spinel compositions from Madawara ultramafics, Bundelkhand Craton: Implications on petrogenesis and tectonic evolution of the southern part of Bundelkhand Craton, Central India

Madawara ultramafic complex (MUC) in the southern part of Bundelkhand Craton, Central India comprises peridotite, olivine pyroxenite, pyroxenite, gabbro, and diorite. Coarse‐grained olivine, clinopyroxene (Cpx), amphibole (Amp), Al‐chromite, Fe‐chromite, and magnetite with rare orthopyroxene (Opx) are common minerals in peridotite. Chromites are usually coarse‐grained euhedral found as disseminated crystals in the olivine matrix showing both homogeneous and zoned texture. Al‐chromite, primarily characterizes Cr‐spinels and its subsequent fluid activity and alteration can result in the formation of Fe‐chromite, chrome magnetite, and magnetite. Mineral chemistry data suggest that Al‐chromite is characterized by moderately high Cr2O3 (38.16–51.52 wt.%) and Fe2O3 (3.22–14.51 wt.%) and low Al2O3 (10.63–21.87 wt.%), MgO (1.71–4.92 wt.%), and TiO2 (0.22–0.67 wt.%), whereas the homogeneous Fe‐chromite type is characterized by high Fe2O3 (25.54–47.60 wt.%), moderately low Cr2O3 (19.56–37.90 wt.%), and very low Al2O3 (0.06–1.53 wt.%). Subsequent alteration of Al‐chromite and Fe‐chromite leads to formation of Cr‐magnetite and magnetite. The Cr# of Al‐chromite varies from 55.12 to 76.48 and γFe3+# from 8 to 19, whereas the ferrian chromite has high Cr# varying from 94.27 to 99.53 while its γFe3+# varies from 38 to 70. As a whole, the primary Al‐chromite shows low Al2O3, TiO2 contents, and high Fe#, Cr# values. Olivines have forsterite ranging from 75.96% to 77.59%. The bulk‐rock geochemistry shows continental arc geochemical affinities indicated by the high concentration of large‐ion lithophile elements and U, Th relative to the low concentration of high‐field strength elements. These petrological and mineralogical as well as primary Al‐chromite compositions plotted in different discrimination diagrams suggest an arc environment that is similar to Alaskan‐type intrusion.

Temporal variations in the mantle source beneath the Eastern Tianshan nickel belt and implications for Ni–Cu mineralization potential

Many small mafic–ultramafic intrusions are found in the Eastern Tianshan Ni belt along the southern margin of the Central Asian Orogenic Belt (CAOB). The Ni–Cu mineralized Hulu intrusion is located in the eastern part of the Jueluotage area of the Eastern Tianshan Ni belt. This mafic–ultramafic intrusion comprises a shallow layered sequence and deeper dike-like unit. Precise zircon U–Pb dating yielded ages of 381.0 ± 1.3 and 377.0 ± 3.0 Ma for sulfide-bearing samples from the layered unit and 388.6 ± 2.7 Ma for a mineralized gabbro from the dike-like unit. 176Hf/177Hf values of zircons from the layered unit range from 0.282846 to 0.283165 and correspond to εHf(t) values of +10.6 to +20.4. δ18O values for these zircons range from +4.8‰ to +5.5‰. Similarly, zircons from the sulfide-bearing gabbro of the dike-like unit have εHf(t) and δ18O values from +13.0 to +18.2 and +5.0‰ to +5.6‰, respectively. Normalized SiO2, Al2O3, TiO2, and Na2O + K2O contents increase with decreasing MgO content, whereas the FeOtotal content is positively correlated with MgO, which indicates that the different lithologies resulted from the fractional crystallization of a common parental magma. Furthermore, the mafic–ultramafic intrusions and their calculated parental magmas in the Jueluotage area have similar rare earth element (REE) patterns with slight enrichment of the light REE relative to heavy REE. All of the rocks and estimated parental magmas exhibit depletions in high-field-strength elements (HFSE; e.g., Nb, Ta, and Zr) and enrichments in large-ion lithophile elements (LILE; e.g., U). The pronounced light REE and LILE enrichments and HFSE depletions of the mafic–ultramafic intrusions and calculated parental magmas are indicative of a subduction-modified mantle source. Clinopyroxenes in the Hulu intrusion have variable Al2O3 (4.39–7.32 wt%) and low TiO2 (0.33–0.67 wt%) contents, which are comparable to typical Alaskan-type complexes that were emplaced in subduction environments. In a TiO2 versus Alz (AlIV × 100/2) diagram, data for the Devonian mafic–ultramafic intrusions in the Jueluotage area fall within the fields of Alaskan-type complexes, whereas data for most of the Permian intrusions fall outside of these fields. Systematic in situ zircon O and Hf isotope analyses from the Devonian mafic–ultramafic intrusions in the Jueluotage area exhibit higher δ18O and lower εHf(t) values than those of the Permian intrusions, which may be explained by a change from a Devonian fluid- to Permian fluid- and melt-modified mantle source. The mafic–ultramafic intrusions that were emplaced in both subduction and post-collisional extensional environments in the Eastern Tianshan Ni belt were favorable hosts to magmatic Ni–Cu deposits, despite their different mantle sources.

Timing and tectonic settings of the Late Paleozoic intrusions in the Zhusileng, northern Alxa: implication for the metallogeny

The Zhusileng–Hangwula tectonic zone, located in the northern Alxa area, is a key site for understanding the evolution of the Paleo-Asian Ocean (PAO) and related mineral formation. We report the major and trace elements and zircon U-Pb ages of newly discovered mafic and intermediate to felsic intrusive rocks in the Zhusileng area. LA-ICP-MS zircon U-Pb analyses of the Echenghei diabase and Haierhan and Heipingshan granites yielded crystallization ages of 305 Ma, 284 Ma, and 271 Ma, respectively. The Late Carboniferous mafic rocks have high MgO and Fe2O3T contents and low K2O and TiO2 contents, and are characterized by depletion of HREEs and notably negative Nb-Ta anomalies, with enrichment of Rb, Ba, and U, which indicate an enriched mantle source. The Early Permian granites of the Zhusileng area are highly differentiated I-type granitoids sourced from mixing of mantle and crust, are high-K calc-alkaline and enriched in LREEs, and exhibit moderately to strongly negative Eu anomalies. All the intrusive rocks formed in a post-collisional extensional environment, which indicates that the branch of the PAO in the northern Alxa area had closed before the Early Permian. The Cu-Mo/Au mineralization in the Zhusileng area formed in the Middle to Late Permian and is genetically linked to the Late Paleozoic post-collisional rifting magmatism from after the closure of the Paleo-Asian Ocean.

Granite zircon U-Pb geochronology and geochemistry and the geological significance of the Saibo copper deposit in the western Tianshan Mountains, Xinjiang Province, China

The discovery of the Saibo copper deposit was a significant breakthrough in prospecting in the western Tianshan Mountains, Xinjiang Province. This deposit has great significance for studying the transboundary metallogenic belt because the deposit is associated with granitoids. Zircon dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yield 206Pb/238U ages of 385.9 ± 1.3 Ma and 387.2 ± 1.4 Ma for the granodiorite and granite porphyries from the deposit, respectively, indicating that the granitoids are the products of Middle Devonian magmatism. In terms of their geochemistry, these porphyries have high SiO2 contents (between 62.60% and 67.47%) and low TiO2 contents (between 0.32% and 0.52%), with aluminum saturation indexes (ASI) between 0.76 and 0.90; these characteristics indicate a metaluminous medium-potassium (K) calc-alkaline I-type granite that is enriched in large-ion lithophile elements such as K and Sr, as well as in incompatible elements such as Th and U and light rare earth elements (LREE); is relatively depleted in high-field strength elements such as Na, Ta, P, and Ti; and lacks an obvious Eu anomaly. The zircon Lu-Hf isotope ratios are relatively high, with 176Hf/177Hf values varying between 0.28253 and 0.28272 and εHf (t) that varies between −0.4 and +6.5 (mostly positive). These features demonstrate that the granites were the products of co-magmatic evolution originating from the depleted mantle and the partially melting of juvenile crustal material, which occurred due to the North Tianshan Ocean subduction towards the southern Yili block that led to magma ascent from the depleted mantle.

Petrogenesis of granite from Xiaofan Mo deposit, Dabie Orogen

The Mesozoic granitoids in the Dabie Orogen are of particular geological interest as indicators for Mesozoic lithospheric evolution because of their close association with porphyry Mo mineralization. Here, we present a study using petrogeochemistry data to constrain the petrogenesis of the Xiaofan granites in the Dabie Mo mineralization belt (DMB), Henan Province, China. Field investigations show that the Xiaofan pluton mainly consists of porphyritic granite. The Xiaofan granites have high SiO2 contents of 74.29 wt%–76.07 wt% (average: 75.18 wt%), Al2O3 contents of 11.66 wt%–12.83 wt% (average: 12.13 wt%), and K2O contents of 5.37 wt%–7.90 wt% (average: 6.86 wt%) and low MgO (0.06 wt%–0.16 wt%), TiO2 (0.09 wt%–0.10 wt%), and P2O5 (0.047 wt%–0.103 wt%) contents. They are enriched in Rb, U, K and Hf but depleted in Ba, Nb, Ta, Sr and Ti. By geochemical and mineralogical features, we propose that the Xiaofan granites belong to A-type type granite and dominantly sourced from the crust. The granites from the Xiaofan Mo deposit may have formed in a post-collision extensional setting.

Petrogenesis of Mesozoic volcanic rocks in the Zha'ertai area, Inner Mongolia: Constraints from trace elements and Sr–Nd–Pb isotopes

The Mesozoic acid volcanic rocks of the Zha'ertai area, located in Inner Mongolia, were formed in the late Early Cretaceous and resulted in polymetallic deposits of gold, silver, and other precious metals with huge commercial value. The more developed acid volcanic rocks include subvolcanic and pyroclastic rocks and volcanic lava, with SiO2 of 73.18–83.82%, Na2O + K2O of 6.17–9.03%, and K2O/Na2O of 1.28–70.27%. These rocks are enriched in large‐ion lithophile elements and high‐field strength elements. They are also enriched in light rare earth elements (REEs), with light to heavy REE ratios of 2.91–14.30; light to heavy REE fractionation is significant, and there are obvious negative Eu anomalies (δEu = 0.02–0.25). The acid volcanic rocks have extremely low TiO2, MgO, P2O5, and TFe2O3 contents, exhibit significant enrichment in Th, U, and Pb, and are strongly depleted in Ba and Sr, showing the trace elements feature of A‐type rhyolite. The rocks have smaller (87Sr/86Sr)i, lower εNd (t), younger TDM2, lower (206Pb/204Pb)i,(207Pb/204Pb)i, and (208Pb/204Pb)i. The acid volcanic rocks differ considerably from the intermediate to basic volcanic rocks in geochemical composition, and there is a compositional hiatus present in the succession. This indicates that neither endmember of these Mesozoic bimodal volcanic rocks is cogenetic and that rhyolitic rocks might not be a product of basaltic magma crystallization differentiation. Instead, they may result from partial melting of crust‐derived rock, stemming from partial melting of rocks in the pre‐existing crust underplated by mantle‐derived magma. This suggests that the acid volcanic rocks were formed in a lithospheric extensional setting during the Early Cretaceous.

2.85 Ga and 2.73 Ga A-type granites and 2.75 Ga trondhjemite from the Zhongxiang Terrain: Implications for early crustal evolution of the Yangtze Craton, South China

The discovery of the Neoarchean Yangpo Group and Jinshan A-type granites in the Zhongxiang Terrain opened a new research window on the early crustal evolution of Yangtze Craton. We present here new zircon dating and detailed geochemical evidences to further reveal the magmatic events and their implication to related tectonic regimes. The combined study of zircon U-Pb ages, Hf isotope compositions, and whole-rock geochemistry were carried out for gneissic granites from the Huachong and Huashanguan complexes. Two groups of granite samples from the Huachong Complex have high SiO2, alkali (K2O + Na2O) and TFeO contents and relatively low Al2O3, CaO, MgO, TiO2 and P2O5 contents; all samples are noticeably peraluminous (A/CNK = 1.21–1.48) with negative anomalies in Nb, Sr, P and Ti, and positive anomalies of HFSE (Zr, Hf, Y) and LILE (Rb, U, Th, La) contents. These samples also show very high zircon saturation temperatures (Tzr (°C) = 910–942 °C), high 104 × Ga/Al (2.62–3.51), and high Zr + Nb + Y + Ce contents (571–767 ppm). Collectively, these observations illustrate typical characteristics of A-type granites. The samples from the Qinchong gneissic granites have high sodic, Sr/Y, Nb/Ta and (La/Yb)N with steep REE patterns, identical to those of TTG (tonalite–trondhjemite–granodiorite). These observations suggest that these rocks were generated by partial melting of juvenile crust materials that left a rutile-bearing eclogite residue. Zircon U–Pb dating for two A-type gneissic granites from the Huachong Complex yields weighted mean 207Pb/206Pb ages of 2856 ± 12 Ma and 2732 ± 18 Ma, and the Qinchong trondhjemite yields a upper intercept age of 2749 ± 46 Ma. Thus, we recognize three types of Archean magmatic events in the Zhongxiang Terrain as follows: ① ca. 2.85 Ga A-type granite, which is the oldest A-type granite currently known in South China; ② ca. 2.75 Ga trondhjemite, which may be resulted from arc-continental collision; ③ ca. 2.73 Ga A-type granite, which may be associated with back-arc extension. Therefore, combined with recent studies conducted in the Zhongxiang Terrain, the ca. 2.65 Ga Jinshan A-type granite in Huji region, these magmatic events suggest that the Yangtze Craton (including major Zhongxiang and Kongling terrains, and other microblocks) might have experienced two tectonic cycles of continental assembly and breakup from the Mesoarchean to Neoarchean era (ca. 2.90–2.85 Ga and ca. 2.75–2.65 Ga).

Silurian magmatism on the eastern margin of the Erguna Block, NE China: Evolution of the northern Great Xing'an Range

There are few Early Paleozoic volcanics and mafic rocks in the Xing'an-Mongolia Orogenic Belt in spite of the extensive occurrence of granites caused by a synchronous orogenic event. Despite the fact that this was a major orogen, the properties of the lithosphere and its associated orogenic processes are poorly known. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes, and whole-rock elemental analyses was carried out on three intermediate-mafic intrusions, which were recently identified in the northern Great Xing'an Range. Representative samples of the Yikete, Yalu, and Xinglong intrusions yielded the weighted mean 206Pb/238U ages of 435 ± 1 Ma, 430 ± 8 Ma, and 447 ± 4 Ma, respectively. This indicates the presence of Silurian intermediate-mafic rocks in the region. The dioritic rocks exhibit a Na-rich, a calc-alkaline affinity and enrichment in LILEs (Rb, Ba, K, and Sr) and depletion in HSFEs (Ta, Ti, Zr, and Hf), indicating that the Yikete dioritic rocks were a mantle wedge that had previously been subjected to metasomatism by fluids and melts related to a subducted oceanic slab and its overlying sediments. The gabbroic rocks are tholeiitic in composition with variable MgO (3.14–8.31%), and low K2O (0.16–1.0%) and TiO2 (0.25–2.12%) contents. They are characterized by enrichment of LILEs, depletion of HFSEs, and highly positive zircon εHf(t) (+5.8 to +13.3) values. In addition, they possess geochemical features of MORB with subordinate OIB. The geochemical features indicate that the gabbroic rocks were derived from an upwelling of asthenospheric mantle that had been into the supra-subduction wedge facilitated by subduction rollback before their generation. Combined with the coeval arc-type magmatic rocks widely exposed on the eastern margin of the Erguna Block, we link the generation of the dioritic rocks to the northwestern subduction of the Nenjiang oceanic plate during the Early Paleozoic. The generation of the gabbroic rocks might result from rifting of the back-arc basin. Consequently, an arc-back-arc model is proposed to explain the subduction evolution of the Nenjiang Ocean during the Early Paleozoic.

The Wulonggou metaluminous A2-type granites in the Eastern Kunlun Orogenic Belt, NW China: Rejuvenation of subduction-related felsic crust and implications for post-collision extension

The Wulonggou Pluton is located in Wulonggou area, eastern segment of the Eastern Kunlun Orogenic Belt, NW China, and consists of mainly alkali-feldspar granites covering an area of about 150 km2. Petrogenesis of these granitoids has been investigated through an integrated study of petrography, zircon UPb ages, whole-rock geochemistry, and HfNd isotopic compositions. UPb dating of magmatic zircons indicated these granites crystallized during 426–424 Ma in the middle Silurian. The granites display high SiO2 (75.26–77.55 wt%), K2O + Na2O (7.98–9.03 wt%), extremely low MgO (0.04–0.19 wt%), CaO (0.28–0.61 wt%), and TiO2 (0.05–0.09 wt%) contents showing metaluminous, calcic-alkali and ferroan features; enrichment in Rb and some HFSEs (Zr, U, Nb, Ta, and Y), depletion in Sr, Ba, P, and Ti, mostly right-inclined REE curve, flat HREE patterns, high 10,000 ∗ Ga/Al and intensively negative Eu anomalies, exhibiting an A2-type granite affinity with Y/Nb > 1.2 mostly. The primitive magma of these large quantities of granites was generated under a high temperature, low pressure, reduced and anhydrous environment indicating intense upwelling of asthenosphere. Combining with the positive uniform zircon εHf(t) values of −0.2 to +3.8 and decoupled εNd(t) values of −4.9 to −2.1 at t = 424 Ma, it can be concluded that subduction-related juvenile materials, probably calc-alkaline granitoids, are the source of these A-type granites. Geochemical studies of Wulonggou granites, spatial and temporal distributions of regional magmatism, metamorphism, and sedimentary records throughout the Eastern Kunlun Orogen Belt jointly indicate that the whole orogenic belt was in a typical post-collision extension setting and experienced an isostatic uplift during the middle Silurian triggered by delamination after the convergence of the northeastern margin of Gondwana.

Zircon U-Pb Ages and Sr-Nd-Hf Isotopic Characteristics of the Huichizi Granitic Complex in the North Qinling Orogenic Belt and Their Geological Significance

The Huichizi granite complex is the largest Paleozoic I-type intrusion located in the North Qinling orogenic belt (NQB). In this study, we present systematic geochemical element data, zircon U-Pb ages, Lu-Hf isotopic data, and Sr-Nd isotopic data for the Huichizi granites. In terms of mineral and chemical compositions, these granites are biotite monzonitic and alkali-feldspar granites, both of which are characterized by high TiO2 and total alkali contents and low MgO, TiO2, and TFeO contents. These granites are weakly peraluminous (A/CNK values are 1–1.06 for biotite monzonitic granites and 1.04–1.09 for alkali-feldspar granites) and possess the geochemical characteristics of adakitic rocks, e.g., high Sr contents (319 ppm–633 ppm), Sr/Y ratios (18.5–174), and (La/Yb)N ratios (17.6–57) and low MgO (0.04 wt.%–0.83 wt.%), Y (3.0 ppm–17.2 ppm), and heavy rare-earth element (HREE) contents. This indicates that these rocks were most likely derived from the partial melting of a thickened lower crust. In situ zircon U-Pb dating of these granites yields Early Caledonian ages (437 Ma for biotite monzonitic granites and 424 Ma for alkali-feldspar granites), indicating that the Huichizi granitic complex is the product of multi-periodic magmatism. The positive but varying zircon eHf(t) values (+0.6 to +8.5) suggest that this thickened lower crust was mainly juvenile, i.e., accreted from depleted mantle during the Neo-Mesoproterozoic Period, but involved the ancient recycled crust. Biotite monzonitic granites formed during crust thickening at the extrusion stage, whereas the alkali granites formed during crust thickening at the extension stage (post extrusion). The Huichizi granite complex witnessed the process of extrusion to extension because of the collision between the NCB and the Qinling microcontinent in the Caledonian.

Petrogenesis of the Harsin–Sahneh serpentinized peridotites along the Zagros suture zone, western Iran: new evidence for mantle metasomatism due to oceanic slab flux

AbstractThe Harsin–Sahneh serpentinized peridotites are widely exposed along the Zagros suture zone in the western region of Iran and are considered to represent remnants of Neo-Tethys oceanic lithosphere at the junction of the Arabian and Iran Plates. These rocks are characterized by low contents of SiO2 (38.8–43.5 wt%), Al2O3 (0.1–3.8 wt%), CaO (0.2–8.2 wt%) and TiO2 (&lt; 1 wt%) and high MgO contents (31.1–46.0 wt%). Their enrichments of large ion lithophile elements and light rare earth elements, with high 87Sr/86Sr(i) values (0.7036–0.7109) and relatively high variations in their εNd(t) (–7.5 to +7.8) values, indicate that the Harsin–Sahneh peridotites were metasomatized by flux released from the oceanic subducting slab in an active margin. The chemical compositions and isotopic ratios of these rocks suggest that they were formed as residue of mid-oceanic ridge basalt in the lithosphere that was then subsequently re-melted and metasomatized in a supra-subduction zone system. The occurrence of both mid-oceanic ridge and supra-subduction zone-type peridotites suggests that the heterogeneity of the upper mantle may have occurred due to the different ratios of partial melting and melt–rock reaction processes in different tectonic settings within the Neo-Tethys realm. The Harsin–Sahneh peridotites provide a good explanation of multistage melt extraction as well as melt–rock and metasomatic reactions in the mantle sequence of the Zagros ophiolite complex.

Petrogenesis of plagiogranites in the Muslim Bagh Ophiolite, Pakistan: implications for the generation of Archaean continental crust

AbstractHigh-SiO2 rocks referred to as oceanic plagiogranites are common within the crustal sequences of ophiolites; however, their mode of petrogenesis is controversial with both late-stage fractional crystallization and partial melting models being proposed. Here, we present new whole-rock data from plagiogranitic dyke-like bodies and lenses from the lower and middle sections of the sheeted dyke complex of the Cretaceous Muslim Bagh Ophiolite, northwestern Pakistan. The plagiogranites have similar geochemical signatures that are inconsistent with them being the fractionation products of the mafic units of the Muslim Bagh Ophiolite. However, the plagiogranites all display very low TiO2 contents (&lt;0.4 wt%), implying that they formed by partial melting of mafic rocks. Melt modelling of a crustal gabbro from the Muslim Bagh Ophiolite shows that the trace-element signature of the plagiogranites can be replicated by 5–10% melting of a crustal hornblende gabbro with amphibole as a residual phase, resulting in a concave-up middle rare Earth element pattern. Compositional similarities between the Muslim Bagh Ophiolite plagiogranites and Archaean TTG (trondhjemite–tonalite–granodiorite) has implications for the generation of juvenile Archaean continental crust. As the Muslim Bagh Ophiolite was derived in a supra-subduction zone, it is suggested that some Archaean TTG may have been derived from melting of mafic upper crust in early subduction-like settings. However, due to the small volume of Muslim Bagh Ophiolite plagiogranites, it is inferred that they can be instructive on the petrogenesis of some, but not all, Archaean TTG.

Carboniferous Arc Setting in Central Hainan: Geochronological and Geochemical Evidences on the Andesitic and Dacitic Rocks

Volcanic rocks in the Bangxi-Chenxing tectonic zone provide important carries for better understanding the Late Paleozoic tectonic evolution in Hainan and its temporal-spatial pattern of the eastern Paleotethyan evolution. This paper presents a set of new geochronological and geochemical data on the andesitic and dacitic rocks along the Bangxi-Chenxing tectonic zone in central Hainan. The representative andesitic and dacitic samples yield similar zircon U-Pb ages of 353±3 and 351±7 Ma, respectively, being of Early Carboniferous origin. These volcanic rocks are characterized by low TiO2 and high Al2O3 contents and are enriched in LILEs and LREEs but depletion in HFSEs, along with negative eNd(t) values of -1.4– -4.7 and high 87Sr/86Sr(i) ratios of 0.707 2–0.710 1. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a metasomatized wedge modified by the slab-derived component in a continental arc setting. In combination with the available data, it is proposed for the development of a Carboniferous continental arc in response to the eastern Paleotethyan evolution. The Bangxi-Chenxing tectonic zone might westerly link with the Jinshajiang-Ailaoshan-Song Ma suture zone, constituting an assemblage boundary between the South China and Indochina blocks.

Tectonic significance of the Late Carboniferous Zhunmubutai ophiolitic mélange from Xi‐Ujimqin, Inner Mongolia

The Zhunmubutai ophiolitic mélange is a newly discovered ophiolite suite located between the Solonker‐Linxi ophiolite belt and the Erenhot‐Hegenshan‐Xi‐Ujimqin ophiolite belt, the central part of Xilinhot Microcontinent (XLMC). It was composed of plagiogranite, pyroxene peridotite, pillow basaltic andesite, and associated mafic rocks and surrounded by a quartz diorite pluton and mélange zone containing sandstones and slates of Early Permian Shoushangou Formation. In this study, zircon grains from a quartz diorite and a plagiogranite yield 206Pb/238U ages of 313 ± 1 Ma (MSWD = 1.60, n = 20) and 328 ± 1 Ma (MSWD = 0.9, n = 93), respectively. Sixty‐four zircons from the slate yielded ages ranging from 307 to 2,313 Ma, and with a peak at 340 Ma. These data indicate the Zhunmubutai ophiolitic mélange formed 300 to 340 Ma. The Zhunmubutai plagiogranites that have low contents of K2O (0.07–0.33 wt.%), K2O/Na2O (0.02–0.09), and Rb/Sr (0.01–0.05) show an oceanic plagiogranites affinity. They are also characterized by low contents of TiO2, Nd, Ta, and significant LREE enrichment. Furthermore, the εHf (328 Ma) values range from 14.23 to 18.87. These features suggest that the plagiogranite was originated by the anatexis of amphibolites and with crustal contamination. Combined with previous studies, suggest that the Zhunmubutai ophiolitic mélange may have formed in an extensional environment during the early stages of opening of an ocean basin.