Petrogenesis and Tectonic Setting of Early Cretaceous Intrusive Rocks in the Northern Ulanhot Area, Central and Southern Great Xing’an Range, NE China

Abundant Early Cretaceous magmatism is conserved in the central and southern Great Xing’an Range (GXR) and has significant geodynamic implications for the study of the Late Mesozoic tectonic framework of northeast China. In this study, we provide new high-precision U–Pb zircon geochronology, whole-rock geochemistry, and zircon Hf isotopic data for representative intrusive rocks from the northern part of the Ulanhot area to illustrate the petrogenesis types and magma source of these rocks and evaluate the tectonic setting of the central-southern GXR. Laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U–Pb dating showed that magmatism in the Ulanhot area (monzonite porphyry: 128.07 ± 0.62 Ma, quartz monzonite porphyry: 127.47 ± 0.36, quartz porphyry: 124.85 ± 0.34, and granite porphyry: 124.15 ± 0.31 Ma) occurred during the Early Cretaceous. Geochemically, monzonite porphyry belongs to the metaluminous and alkaline series rocks and is characterized by high Al2O3 (average 17.74 wt.%) and TiO2 (average 0.88 wt.%) and low Ni (average 4.63 ppm), Cr (average 6.69 ppm), Mg# (average 31.11), Y (average 15.16 ppm), and Yb (average 1.62 ppm) content with enrichment in Ba, K, Pb, Sr, Zr, and Hf and depletion in Ti, Nb, and Ta. The granitic rocks (e.g., quartz monzonite porphyry, quartz porphyry, and granite porphyry) pertain to the category of high-K calc-alkaline rocks and are characterized by high SiO2 content (>66 wt.%) and low MgO (average 0.69 wt.%), Mg# (average 31.49 ppm), Ni (average 2.78 ppm), and Cr (average 8.10 ppm) content, showing an affinity to I-type granite accompanied by Nb, Ta, P, and Ti depletion and negative Eu anomalies (δEu = 0.57–0.96; average 0.82). The Hf isotopic data suggest that these rocks were the product of the partial melting of juvenile crustal rocks. Notably, fractionation crystallization plays a crucial role in the process of magma emplacement. Combining our study with published ones, we proposed that the Early Cretaceous intrusive rocks in the Ulanhot area were formed in an extensional tectonic background and compactly related to the subduction of the Paleo-Pacific Ocean plate.

The Metallogeny of the Tieling Cu-Mo Porphyry Deposit in Eastern Tianshan, NW China: New Insights from Zircon U-Pb, Fluid Inclusion, and H-O-S Stable Isotope Analyses

The eastern Tianshan metallogenic belt is an important molybdenum resource base in Xinjiang and is characterized by large-scale porphyry Mo deposits formed during the Triassic. The Tieling Cu-Mo porphyry deposit, which is situated in the western part of the eastern Tianshan metallogenic belt, was recently recognized as being related to Carboniferous granite porphyry. Three stages of hydrothermal mineralization were identified, including quartz+K-feldspar+pyrite±molybdenite±magnetite (stage I), quartz+molybdenite+pyrite+chalcopyrite (stage II), and quartz+pyrite±molybdenite±epidote (stage III). Fluid inclusion petrography and microthermometry analyses indicate the presence of gas-liquid inclusions with a H2O-NaCl composition. The ore-forming fluids have a characteristic temperature ranging from 157 to 262°C under stage II and 135 to 173°C under stage III, which correspond to salinities of 7.2-17.2 wt% NaCl equiv. and 5.9 to 9.6 wt% NaCl equiv., respectively. The hydrogen and oxygen isotope data indicate that the ore-forming fluids of the Tieling deposit were originally derived from magmatic hydrothermal fluids and then mixed with meteoric water. The sulfur isotope compositions indicate that the ore-forming materials were mainly derived from the Late Carboniferous felsic magma. Furthermore, zircon U-Pb analysis of ore-bearing granite porphyry yields a concordant age of 298.4 ± 0.7 Ma , indicating that the Tieling Cu-Mo deposit formed during the Late Carboniferous and differed from that processed under pre-Early Carboniferous and Triassic mineralization in the eastern Tianshan metallogenic belt. These results also indicate that the Tieling porphyry deposit was formed in the transition condition between subduction-related accretion and postcollisional orogeny, and it should be given more attention in prospect evaluations.

Decoupling of Sr-Nd Isotopic Composition Induced by Potassic Alteration in the Shapinggou Porphyry Mo Deposit of the Qinling–Dabie Orogenic Belt, China

In our previous study on petrogenesis of quartz syenite and granite porphyry, the host rocks of the Late Mesozoic Shapinggou Mo deposit in the Qinling–Dabie orogenic belt, we found that the initial Sr isotopic composition of the host rocks is strongly affected by the degree of K-alteration. Here, we provide further isotopic evidence of the host rocks and their minerals to investigate the geochemical behaviour of trace elements and isotopes during the alteration and to explain the phenomenon of decoupling of Sr–Nd isotopic composition. The quartz syenite and granite porphyry are altered by K-alteration in varying degrees and have high K2O and Rb contents and low Na2O, CaO, Sr, and Ba contents. Rock samples of both quartz syenite and granite porphyry have variable Rb/Sr ratios and initial 87Sr/86Sr values (even < 0.70) but contain quite homogeneous εNd(t) values (−12.8 to −14.8). Minerals from the rocks of moderate to intense K-alteration have very low initial 87Sr/86Sr values (even < −17), while those from the weakly altered rocks have 87Sr/86Sr(t) values of 0.7044 to 0.7084. The same phenomenon of the decoupling in Sr–Nd isotopic composition can be observed from several Mo deposits within the eastern Qinling–Dabie orogenic belt. This fact suggests similar hydrothermal features and a comparable origin for both the magmatic rocks and hydrothermal fluids in this belt. A comparison between porphyry Mo and porphyry Cu deposits shows that elements and the Rb–Sr isotope system have different behaviours during the K-alteration, implying distinct material sources and igneous rocks for porphyry Mo and porphyry Cu deposits, respectively.

Unraveling the link between mantle upwelling and formation of Sn-bearing granitic rocks in the world-class Dachang tin district, South China

Increasing evidence shows that the mantle contributes (directly or indirectly) to Sn-bearing granites worldwide. However, the specific role of mantle in the formation of tin granites and related mineralization remains poorly understood. In the world-class Dachang district, South China, tin mineralization is related to the Longxianggai equigranular/porphyritic biotite granites and tin orebodies are cut by granite porphyry dykes hosting mafic microgranular enclaves (MMEs). A combination of zircon U-Pb dating and Hf-O isotopes, mineral chemistry, and whole-rock elemental and Sr-Nd isotopic compositions—for granitic rocks and MMEs, is employed to constrain the petrogenesis and to unravel the link between tin fertility and mantle upwelling. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating indicates that the biotite granites were emplaced at ca. 93 Ma, and the granite porphyry dykes and MMEs were formed at ca. 86 Ma. The biotite granites are silica- and alkali-enriched with A/CNK ratios of 1.04−1.36, and exhibit elevated concentrations of Li, F, P, Rb, Cs, Ta, Sn, W, and U, showing affinities with highly fractionated S-type granites. Whole-rock geochemical and Nd isotopic (εNd(t) = −10.0 to −7.8) data, and in situ zircon Hf-O (εHf(t) = −9.9 to −3.9, δ18O = 6.2−8.9‰) isotopes indicate that the biotite granites were formed by partial melting of metasedimentary rocks at relatively high temperatures (≥782 °C), possibly with minor input of mantle material. Likewise, the post-ore granite porphyry dykes have similar chemical and mineralogical characteristics as fractionated S-type granites. Zircon Hf-O isotopes (εHf(t) = −9.0 to −4.9, δ18O = 6.5−8.2‰) and whole-rock geochemical data suggest they were derived from a similar source as the biotite granites, whereas elevated εNd(t) values of −5.0 to −3.3 for granite porphyry dykes relative to biotite granites reveal an increasing mantle input. Distinct εNd(t) (−0.4 and −0.3) and zircon Hf-O (εHf(t) = 1.5−5.0, δ18O = 6.5−7.2‰) isotopes of the MMEs, suggest that the mafic melt could be sourced from the asthenospheric mantle, contaminated by subcontinental lithospheric mantle/continental crust during magma ascent, and hybridized by felsic melt at emplacement-level. The magmatic sequence in the Dachang district is indicative of an extensional tectonic setting where mantle-derived magmas are predicted to migrate to shallower crustal levels as the crust progressively becomes thinner and hotter. High-temperature partial melting of mature metasedimentary crust triggered by heat input from the upwelled mantle, may contribute to biotite breakdown, which is important for concentrating tin in melts. Fractional crystallization of initially Sn-rich felsic melts under reduced conditions makes further tin enrichment and produces Sn-bearing granites (the Longxianggai pluton). Prolonged mantle upwelling results in distinct magma mixing and the formation of granite porphyry dykes and MMEs. These dykes are highly fractionated with elevated Sn and W contents, which show great potential to form hydrothermal Sn-W mineralization.

Geochronology, Geochemistry, and Geodynamic Relationship of the Mafic Dykes and Granites in the Qianlishan Complex, South China

The Qianlishan complex, located in Hunan Province of South China, is closely associated with intense W-dominated polymetallic mineralization. The Qianlishan complex is composed of three phases: the main-phase porphyritic and equigranular granites, granite porphyry, and mafic dykes. Geochronologically, the zircon U-Pb dating results show that the porphyritic and equigranular granites have ages of approximately 159 and 158 Ma, respectively, similar to those of mafic dykes (approximately 158 Ma), while the granite porphyry was formed later at approximately 145 Ma. Geochemically, the mafic dykes are characterized by calc-alkaline high-Mg andesite (HMA) with high MgO, TiO2, Mg#, and CA/TH index. They exhibit significantly depleted εNd(t) and εHf(t) with high Ba/La, La/Nb, and (La/Yb)N, indicating that they formed from mixing melts of depleted asthenospheric mantle and metasomatized subcontinental lithospheric mantle (SCLM). The main-phase granites are peraluminous and are characterized by high SiO2, low (La/Yb)N ratios, and relative depletion in Ba, Sr, Ti, and Eu. They also display negative correlations between La, Ce, Y, and Rb contents, suggesting that they are highly fractionated S-type granites. Furthermore, they show high εNd(t) and εHf(t), CaO/Na2O ratios, HREE, and Y contents, indicating that they were produced by parental melting of ancient basement mixed with mantle-derived components. In contrast, the granite porphyry shows A-type signature granites, with higher εNd(t) and εHf(t) and CaO/Na2O ratios than the main-phase granites but similar Zr/Nb and Zr/Hf ratios to the mafic dykes, suggesting that they are the products of partial melting of a hybrid source with ancient basement and the mafic dykes. We thus infer that the slab roll-back led to generation of Qianlishan back-arc basalt and HMA and further triggered the formation of the Qianlishan granite.

The granite porphyry hidden in the Shuangjianzishan deposit, southern Great Xing’an Range, NE China: geochronology, isotope geochemistry and tectonic implications

The Shuangjianzishan vein-type Ag-Pb-Zn deposit in the southern Great Xing’an Range (GXR), NE China, is hosted in the slate of the Lower Permian Dashizhai Formation intruded by granite porphyry. In this paper, U–Pb zircon ages and bulk-rock and isotope (Sr, Nd, Pb and Hf) compositions are reported to investigate the derivation, evolution and geodynamic setting of this granite porphyry. It is closely associated with Pb-Zn-Ag mineralization in the southern GXR and contains important geological information relating to regional tectonic evolution. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) zircon U–Pb dating yields an emplacement age of 131 ± 1 Ma for the granite porphyry. Bulk-rock analyses show that the Shuangjianzishan granite porphyry is characterized by high Si, Na and K contents but low Mg and Fe contents, and that the enrichment of Zr, Y and Ga suggests an A-type granite affinity. Most of the studied samples have relatively low 87Sr/86Sr values (0.70549–0.70558), with positive ϵNd(t) (0.71–0.88) and ϵHf(t) (4.9–6.9) values. The Sr–Nd isotope modelling results, in combination with the young TDM2 ages of Nd and Hf (850–864 and 668–778 Ma, respectively), reveal that the Shuangjianzishan granite porphyry may be derived from the melting of mantle-derived juvenile component, with minor lower crustal components; this finding is also supported by Pb isotopic compositions. Considering the widespread presence of granitoids with coeval volcanic rocks and regional geology data, we propose that the Shuangjianzishan granite porphyry formed in a post-orogenic extensional environment related to the upwelling of asthenospheric mantle following the closure of the Mongol–Okhotsk Ocean.