Formation Mechanism And Tectonic Setting Of Intermediate-Acidic Masses In Zhashui-Shanyang Area,Southern Qinling

A series of intermediate-acidic small granite (porphyry) masses are widely exposed in the Zhashui-Shanyang region, South Qinling orogenic belt, and most of them are universal developed mineralization. Base on the field geology, petrography, zircon U-Pb geochronology and geochemistry of these granite (porphyry) masses, this paper study on the genetic type of rocks, magmatic source, tectonic evolution and geodynamic setting. The main achievement are listed as follows:The intermediate-acidic granite (porphyry) bodys mainly includes monzogranite (porphyry),granodiorite (porphyry), tonalite, diorite, diorite-porphyrite. Major minerals are quartz, K-feldspar, plagioclase, biotite, amphibole etc, and contain accessory minerals like apatite, zircon, sphene, magnetite etc.LA-ICP-MS zircon U-Pb dating indicates that the weighted mean ages are Xiaguanfang granodiorite (porphyry) (146.78±0.91Ma), Yuanzijie monzogranite (porphyry) (145.56±0.95Ma), Yuanjiagou monzogranite (143.67±0.90Ma), Xiaohekou monzogranite (142.34±0.92Ma), Baishagou quartz diorite (141.09±0.72Ma), Chigou tonalite (141.5±1.0Ma) and Tudigou monzogranite (porphyry) (139.29±0.90Ma), respectively. These ages suggest that granite (porphyry) masses formed in the Early Jurassic-Late Cretaceous.Geochemical studies suggest that plutons has high SiO2 (60.19-69.20 wt.%), alkali (K20+Na207.00～10.21 wt.%), Al2O3 (Al2O3=15.03～15.91 wt.%) contents, low CaO (CaO=1.34-4.54 wt.%) contents, belongs to the high-K calc-alkaline series, they also display a alumina saturation index A/CNK=0.82-1.12, belonging to metaluminous and peraluminous. Chondrite-normalized REE exhibit enrichment of LREE (LREE=103.80～191.95ppm) flat HREE (HREE=8.05-15.04ppm) with slightly negative Eu anomalies (8Eu=0.89～1.10), as well as manifest differentiation of REE (LREE/HREE=10.83～14.89, (La/Yb)N=12.35～21.06). Primitive mantle-normalized trace elements multi-element variation diagrams shows obvious depletion in HFSE such as Nb, Ta, Ti, P, etc. and relative enrichment in LILE such as K, Rb, Sr, Ba, Pb, etc.Studies of field geology, petrography, geochronology and geochemistry indicate that, plutons belong to I-type granite with similar magmatic source mainly derived from themelting of the ancient crust and small crust and mantle mixture. Granite (porphyry) masses formed under the tectonic setting from extrusion, Extrusion makes the crust thickened and its occured melted, lastly,The small granitic plutons is produced.