Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U-Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite （218±1 Ma）. The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with （La/Yb） N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 （mean= 0.97）. Most of these samples from Xiba granitic pluton exhibit εNd（t） values of 8.79 to 5.38, depleted mantle Nd model ages （T DM ） between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios （ 87 Sr/ 86 Sr） i from 0.7061 to 0.7082, indicating a possible Meso-to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher （ 87 Sr/ 86 Sr） i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested th
The Hejiazhuang pluton is located in the South Qinling Tectonic Belt （SQTB） in the north side of the MianxianLueyang Su ture Zone, and consists dominantly of granodiorites. LAICPMS zircon UPb dating and LuHf isotopic analyses reveal that these granodiorites of the Hejiazhaung pluton emplaced at 248 Ma, and show a large variation in zircon eHt（t） values from 4.8 to 8.8. These granodiorite samples are attributed to highK to midK calcalkaline series, and characterized by high SiO2 （66.6±70.0%）, A1203 （15.04±16.10%） and Na20 （3.74±4.33%） concentrations, with high Mg# （54.2±61.7）. All samples have high Sr （627±751 ppm）, Cr （553±73 ppm） and Ni （17.2182 ppm）, but low Y （5.42-8.41 ppm） and Yb （0.59-0.74 ppm） concentrations with high Sr/Y ratios （84.90±120.66）. They also display highly fractionated REE patterns with （La/Yb）N ratios of 18.93-4.0 and positive Eu anomalies （0＂Eu=1.102.22） in the chondritenormalized REE patterns. In the primitive mantle normalized spidergrams, these samples exhibit enrichment in LILEs but depletion in Nb, Ta, P and Ti. These geochemical fea tures indicate that the granodioritic magma of the Hejiazhuang pluton was derived from the partial melting of hybrid sources comprising the subducted oceanic slab and sediments, and the melts were polluted by the mantle wedge materials during their ascent. The emplacement ages and petrogenesis of the Hejiazhuang pluton prove that the initial subduction of the Mianlue oceanic crust occurred at 248 Ma ago, and the SQTB was still under subduction tectonic setting in the Early Triassic.
YANG PengTaoLIU ShuWenLI QiuGenWANG ZongQiZHANG FanWANG Wei
The Zhashui-Shanyang district is one of the most important sulfide deposits in the Qinling Orogen where the formation of porphyry-skarn Cu-Mo deposits has a close genetic link with the Yanshannian magmatism.Laser Ablation-Inductively Coupled Plasma Mass Spectrometry（LA-ICP-MS） U-Pb zircon dating of two granodiorite intrusions（Xiaohekou and Lengshuigou deposits）was investigated in the Zhashui-Shanyang district and the rock-forming ages obtained from 148.3±2.8 to 152.6±1.2 Ma,averaging 150.5 Ma,accompanied by a younger disturbance age of 144.3±1.7 Ma in the Lengshuigou intrusion,which is in excellent agreement with published sensitive high resolution ion micro-probe（SHRIMP）zircon date on the later monzodiorite porphyry phase in the Lenshuigou deposit.Two samples were selected for molybdenite ICP-MS Re-Os isotopic analyses from the Lengshuigou granodiorite porphyry,yielding Re-Os model ages from 149.2±2.7 Ma to 150.6±3.4 Ma, with a weighted mean age of 149.7±2.1 Ma.These mineralization ages overlap rock-forming ages of the host intrusions within the error range.This implies that the mineralization occurred in the Late Jurassic,which belongs to the tectonic phase B event of the Yanshan Movement,not Cretaceous as previously thought.Therefore,the Late Jurassic mineralization of the Zhashui-Shanyang district could be connected to the large-scale Yanshan molybdenum metallogenic period,the geodynamic regime of which is attributable to the far field response of convergence of surrounding plates,perhaps the approximately westward subduction of the Izanagi plate beneath the Eurasian continent.
LI QiugenLIU ShuwenWANG ZongqiWANG DongshengYAN ZhenYANG KaiWU Fenghui
