Petrogenesis Of Strongly Peraluminous Granites In Markan Area, Songpan Fold Belt And Its Tectonic Implication

The Songpan fold belt, located in the easten part of the Tibetan Plateau, is confined by the North China, South China (Yangtze) and North Tibet (Qiangtang) blocks. The belt is a folded Triassic flysch basin which is a huge triangular area with an area of more than 200,000 km2. In the Songpan fold belt, Indosinian post-collision granitoids, including adakitic, A-type and I-type granitoids, are widespread. However, studies on Indosinian strongly peraluminous granite in this area are rare.This paper carries out a study of U-Pb dating, geochemical and Sr-Nd-Hf isotopic compositions from the Markan granites. Some conclusions are sumarried as follows.1. The Markan granites, including medium-grained two-mica granite and medium-fine grained two-mica granite, are strongly peraluminous, with Al index (A/CNK)=1.10-1.20. Zircons from the medium-grained two-mica granite and the medium-fine grained two-mica granite granite are colourless. Zircon CL images show clear magma rhythmic zoning. Some zircons of the Markan granites display inherited zircon cores. Most analyses are concordant. The zircons from the medium-grained two-mica granite yields 206Pb/238U ages ranging from 204 to 214Ma with a weighted mean age of 208±2Ma(MSWD=2.8), which represents the magma crystallization age. Zircons from the medium-fine grained two-mica granite display 206Pb/238U ages ranging from 197 to 210 Ma, with a weighted mean age of 200±2Ma(MSWD=1.6), which represents the magma crystallization age of the medium-fine grained two-mica granite granite. Our data indicate that the magma crystallization age of the Markan granites is 208-200 Ma.2. The medium-grained two-mica granite shows SiO2=73.46%-74.74%, K2O/Na2O=1.13-1.75, Corundum(C)= 0.91%-3.01%. Most samples have A/CNK=1.10-1.20, indicating that they are strongly peraluminous.The medium-fine grained two-mica granite displays SiO2=69.57%-73.70%, K2O/Na2O=1.35-1.71, Corundum (C)= 1.46%-1.85% and A/CNK= 1.08-1.12, also indicating that they are strongly peraluminous. Both of them lie in a field of high-potassic calc-alkaline. Trace elemental data show that the Markan strongly peraluminous granites are enriched in Rb,Th,U and depleted in Sr,Ba,Co and Ni. The medium-grained two-mica granite displays higher Rb and lower Th, Sr and Ba than the medium-fine grained two-mica granite. These characteristics are distinct from A-type,I-type and M-type granitoids, but are similar to S-type granitoids. In rare earth element (REE) compositions, both the medium-grained medium-fine grained two-mica granites display strongly fractionated REE patterns with (La/Yb)N= 6.08-51.84, Eu/Eu*= 0.15-0.44 for the medium-grained two-mica granite and Eu/Eu*= 0.50-0.65 for the medium-fine grained two-mica granite.3. The medium-grained two-mica granite has initial Sr isotopic ratios (ISr) of 0.70712-0.71137 andεNd(t) values of-10.4--8.4, the single-stage Nd depleted-mantle model ages (TDm1) range from 1.70 to 6.77Ga, the two-stage Nd depleted-mantle model ages (TDM2) range from 1.69 to 1.81Ga. The medium-fine grained two-mica granite has ISr of 0.70581-0.70804 andεNd(t) values of-3.73 to-4.72, TDM1=1.64-1.69Ga, TDM2=1.69-1.76Ga The Sr-Nd isotopic composition of the Markan granites provides an additional eveidence that both the granites were derived from crustal source and it is agreement with occurrence of abundant inherited zircons. The medium-grained two-mica granite has 176Hf/177Hf from 0.282314 to 0.282618 and 176Lu/177Hf from 0.000146 to 0.001464, zirconεHf(208) values of-11.8--1.1 with a weighted mean value of -9.1±1.4. The medium-fine grained two-mica granite has 176Hf/177Hf from 0.282445 to 0.282584 and 176Lu/177Hf from 0.000709 to 0.001788, zirconεHf(200) values of-7.4--2.4 with a weighted mean value of -4.1±0.8. The two-stage Hf model ages of medium-grained two-mica granite and medium-fine grained two-mica granite are 1.3-2.0Ga and 1.4-1.7Ga, respectively. Hf isotopic data suggest that the magmas for the Markan granite were mainly derived from patial melting of crust.4. Geochemical and Sr-Nd-Hf isotopic compositions suggest that the magmas for the medium-grained and the medium-fine grained two-mica granites were derived from patial melting of argillaceous and greywacke sediments, respectively. Combined with geological background, regional tectono-magmatic events and magmatism association, Indosinian lithospheric delamination can account for the magma generation of the Markan strongly peraluminous granites. The Indosinian lithospheric delamination resulted in mantle asthenospheric upwelling, which promoted not only partial melting of the thickened lower crust (e.g. the adakitic andⅠ-type granitoid magma gerneration), but also partial melting of the middle-lower crust (e.g. the Markan granite gerneration). This indicates that the Indosinian lithospheric delamination in the Songpan fold belt had resulted in crustal partial melting at different crustal levels.