Chronology And Genesis Of The Lvjing And Baijiazhuang Granitoid Pluton From The West Qinling

The West Qinling orogenic belt,is the western segment of the Qinling orogenic belt with voluminous Indosinian granitoids.Both Lvjing and Baijiazhuang plutons,mainly intruded into the Devonian Xihanshuiqun and Mesoproterozoic Wujiashan Group,are the members of the "Five Golden Flower" granitic plutons group of this period in the Western Qinling.Lvjing pluton is located in the western part of "Five Golden Flower" granitic pluton group,and Baijiazhuang in its centre.Lvjing pluton mainly constitutes biotite monzogranites.While the Baijiazhuang include biotite monzogranites and two-mica monzogranites.The zircon LA-ICP-MS dating for the porphyroid biotite monzogranites,coarse-grained biotite monzogranites and medium-fine-grained biotite monzogranites from Lvjing pluton yielded ages of 221±1Ma(N=20,MSWD=0.11),221±1Ma(N=24,MSWD=0.04)and 218±1Ma(N=15,MSWD=0.13),respectively.These ages suggest that they were formed during Late Triassic and the porphyroid biotite monzogranites formd earlier than medium-fine-grained biotite monzogranites.The zircon LA-ICP-MS dating for the fine-grained biotite monzogranites,medium-fine-grained two-mica monzogranite,fine-grained two-mica monzogranite for Baijiazhuang pluton yielded ages of 216±1Ma(N=14,MSWD=0.06),210±2Ma(N=13,MSWD=0.15)and 217±1Ma(N=17,MSWD=0.42),respectively,also suggestting that they were formed during Late Triassic.Femic minerals of the granitic rocks of the Lvjing pluton are mainly biotite and occasionally contain hornblende,and the accessory minerals are apatite,zircon and titanite.Magmatic microgranular enclaves of this pluton are mainly occurred in the margin of the pluton.This feature is similar to the Zhongchuan pluton.Two-mica monzogranite from Baijiazhuang pluton contain aluminium minerals,such as muscovite and garnet but scarcely contain magmatic microgranular enclaves.Lots of pegmatite veins and quartz-veins occur in this pluton.The rocks from the Lvjing pluton are of high-K calc-alkaline to shoshonite series and belong to weakly peraluminous to peraluminous I-type granites with A/CNK varying from 1.02 to 1.16,Na2O+K2O 7.65%to 8.84%and K2O/Na2O 1.37 to 1.82.The rocks from Baijiazhuang pluton are of high-K calc-alkaline to shoshonite series and belong to peraluminous to peraluminous granites,characterized by w(SiO2)=72.38%?74.61%,w(Na2O+K2O)=8.07%?8.87%,poor calcium,magnesium,and iron,high Rb/Sr(2.0?47.1),A/CNK(1.07?1.21)and K20/Na2O(1.23?1.78)values,and w(Al2O3)(13.89%?14.88%)contents.All of these features suggest that the Baijiazhuang granites are highly fractionated I-type granites.The fractionaton between LREE and HREE is obvious both for the Lvjing and Baijiazhuang granites.Their REE and trace elements are characterized by enriched in Rb,Th,K,depleted in Ba,Sr,P and Ti and with strong negative anomalies of Eu.Compared with the granites of the Lvjing pluton,Baijiazhuang granites demonstrate more negative anomalies of Eu.Also,some samples of Baijiazhuang granites display tetrad effect of REE.Crystallization temperature by zircon thermometer for the Lvjing granites is 736?to 818?,and Baijiazhuang 629? to 798?.The whole rock ?Nd(t)values of porphyroid biotite monzogranite and medium-fine-grained biotite monzogranite from the Lvjing pluton are-7.90 and,-8.20,respectively,corresponding Nd model ages of 1.64Ga and 1.66 Ga.Their zircon ?Hf(t)values are-21.33?-0.36 and mainly between-4?-1,occasionally with positive ?Hf(t)value(+0.6),corresponding two-stage model ages mainly of 1.3-1.6Ga.The whole rock sNd(t)values of Baijiazhuang pluton are-9.44,-9.12 and-8.58,corresponding Nd model ages1.76Ga,1.73Ga and1.69Ga,respectively.Their zircon ?Hf(t)values are-16.03?-2.36 and mainly between-11?-3,occasionally with positive ?Hf(t)value(+2.48),with two-stage model ages mainly being 1.5?1.9Ga.Geochemistry and Nd-Hf isotopic compositions of the granites suggest that it was probably derived from the Mesoproterozoic continental crust and the Paleoproterozoic-Mesoproterozoic continental crust for the Lvjing pluton and Baijiazhuang pluton,respectively,but with juvenile components contribution.The Lvjing pluton is similar to the Zhongchuan pluton in chronology,petro-geochemistry,petrogenesis,isotopic compositions and magmatic evolution.It may imply that there are also potential for gold mineralition surrounding this pluton as that of the Zhongchuan pluton.While compared with the Lvjing and Zhongchuan,the Baijiazhuang pluton has certain particularity in petrography and petrogeochemistry,its highly fractionated geochemical characteristics may have relationship with tungsten and tin mineralition.