Zircon Hf Isotopes and Geochemistry of the Early Paleoproterozoic High-Sr Low-Y Quartz-Diorite in the Quanji Massif,NW China:Crustal Growth and Tectonic Implications

The metamorphosed Early Paleoproterozoic granitoids in the Quanji massif, Northwest- ern China provide constraints for relationship between the Tarim Craton and North China Craton. Among granitoids batholiths, rocks of the Mohe quartz-diorite show typically adakitic geochemical characteristics, with medium K2O/Na2O ratios （0.56-1.17） and high Sr （519-619 ppm） low Y （9.37-20.40 ppm） and Yb （0.97-1.77 ppm） concentrations. The rocks have εNd（t） values between ＋2.4 and ＋4.4 and depleted mantle Nd model ages of 2.43-2.59 Ga. The magmatic zircons have positive εHf（t） values ranging from ＋0.40 to ＋7.60 and depleted mantle Hf model ages of 2.43-2.70 Ga, with major peaks at -2.54 and -2.65 Ga. The geochemical and Nd-Hf isotopic characteristics indicate that the Mohe quartz-dioritic rocks might be formed by partial melting of high-pressure metamorphosed juve- nile crustal rocks in post-orogenic extensional regime in the Early Paleoproterozoic. It suggests that important crustal growth occurred in the Quanji massif and the Tarim Craton at -2.4 and 2.5-2.7 Ga. The Quanji massif and Tarim Craton might share a similar crustal evolution history with the North China Craton in the Neoarchean.

Petrogenesis and Ore Genesis of the Late Yanshanian Granites and Associated Porphyry-Skarn W-Mo Deposits from the Yunkai Area of South China： Evidence from the Zircon U-Pb Ages, Hf Isotopes and Sulfide S-Fe Isotopes

There are a wide range of magmatism and mineralization in the Yunkai area of South China during the Late Yanshanian Period, including the newly discovered Michang, Youmapo, Sanchaehong and Songwang porphyry-skarn W-Mo deposits. In this study, we obtained zircon U-Pb ages of the ore-bearing biotite granites and their mafic enclaves from 884-1 to 1104-1 Ma. Zircons from the granites show Hf isotopic compositions with negative C-Hf（t） values of -5.9 to -0.6 and calculated Hf model ages （TvM2） of 1.5-1.2 Ga; indicating that the Middle Proterozoic crustal materials may have provided an important source for the magmatic rocks in this district during the Late Yanshanian Period, whereas zircons from the mafic enclaves show positive era（t） values of 1.3 to 10.1 with younger Hf model ages （TDM2） of 0.5-1.1 Ga, suggesting a mantle component may have involved in the granitic magma generation. Sulfur isotope study of the sulfide minerals from the W-Mo deposits show a narrow 834S distribution with most data ranging from -4.2%0 to 5.2‰. In addition, this study reports the first Fe isotopic compositions of pyrite in the W-Mo deposits, which show a uniform distribution range with the values near zero （656Fe=0.16‰-0.58‰, average 0.35‰; 657Fe=0.02‰-0.54‰, average 0.48‰）. These data indicate that the ore-forming materials may come from the deep-sourced granitic magma, and the mineralizations show a close relationship with the granitic magmatism during the Late Yanshanian Period. Combining with previous results, we suggest that there is a widespread porphyry-skarn W-Mo mineralization in the Yunkai area during the Late Cretaceous （80-110 Ma）, which has a close relationship with the Late Yanshanian magmatism that may have formed during the rollback of the subducted Pacific Plate.

Geochemistry of Metamafic Dykes from the Quanji Massif: Petrogenesis and Further Evidence for Oceanic Subduction, Late Paleoproterozoic, NW China

A suite of ~1.84-1.92 Ga metamafic dykes within the paragneiss suite（khondalite） of the Quanji massif in NW China, has been chosen in this study for further understanding the tectonic evolution and possible links to the global Columbia supercontinent. Occurrence and field relations suggest that they were formed coevally with a previous studied ~1.83-1.85 Ga metamafic dyke swarms. Whole-rock major and trace elemental geochemistry suggests precursor magma of the amphibolites being generated from a volcanic arc-related tectonic setting rather than a back-arc environment where the metamafic dyke swarms were emplaced. The metamafic dykes show enrichment of LREE and strongly negative anomalies for Ta-Nb, Zr-Hf and Ti, have high SiO_2（49.3 wt.%-52.5 wt.%） but low MgO（6.40 wt.%-7.76 wt.%） contents and Mg~#（Mg~#=[100×（MgO/40.3）]/[MgO/40.3＋FeO/71.8]） values（45.7-52.1）, suggesting evolved precursor magma. The high values of La/Ta（22.2-42.8） and La/Nb（1.71-2.47）, mildly negative εNd（t） values（-2.51-0.15）, with depleted mantle model ages（TDM） of 2.45-2.84 Ga, suggest that their precursor magmas were possibly derived from a subduction-related fluid metasomatized Archean sub-continental lithospheric mantle. This study provides further evidence for oceanic plate subduction prevailing before or around ~1.85 Ga, which confirms a prolonged subduction-accretion-collision history in the NW China which is possibly linked to the assembly of the Columbia supercontinent.

Early Paleoproterozoic Post-Collisional Basaltic Magmatism in Quanji Massif:Implications for Precambrian Plate Tectonic Regime in NW China

Basaltic magmas can provide important information about mantle source nature,tectonic settings and tectonic evolution for a given terrain.This paper reports geology,petrography and geochemistry of whole-rock major and trace elements and Nd-Sr isotopes for a suite of garnet amphibolites from southeastern Wulan(Ulan),Quanji Massif,northwestern China.The garnet amphibolites were likely generated from basaltic lavas,associated with both paragneisses and orthogneisses of the lower Delingha Group.The basaltic protolith of these amphibolites can be broadly constrained to be formed at~2.33 Ga in an extensional setting post-collision.The geochemistry of amphibolites shows subalkaline and highly evolved characteristics.They display high-Fe low-Ti characteristics,with TFeO of 13.1 wt.%-17.9 wt.%and TiO_(2) of 1.42 wt.%-3.09 wt.%(in most samples TiO_(2)≤2.5 wt.%).The chondrite-normalized REE patterns show enrichment of LREE and LILE and the primitive-mantle-normalized incompatible element patterns display negative P,Ti,Nb-Ta and Zr-Hf anomalies.The(^(87)Sr/^(86)Sr)t values of 0.6978-0.7123 andε_(Nd)(t)values of-2.81-5.08 respond to depleted mantle model ages(T_(DM))of 2.33-3.30 Ga.These suggest that the precursor magmas of the protolith of the garnet amphibolites were probably derived from the Early Paleoproterozoic depleted sub-continental lithospheric mantle that had been metasomatized by subduction-induced fluids and melts.The precursor basaltic magmas were contaminated by the older crustal components during magma ascending.This post-collisional basaltic magmatic event at~2.33 Ga in Quanji Massif thus enhanced the subduction shutdown or slowdown tectonic regime both in NW China and coevally with those plate tectonics in some important domains worldwide during the Early Paleoproterozoic.