Petrogenesis Of Intermediate-basic Igneous Rocks And Cherts From Yaziquan, Xinjiang Qimantag Mountain

The East kunlun orogenic belt is an important part of the Tibet Plateau.The Qimantag Mountain, which has a long research history, located in the western part of the East kunlun orogenic belt. Due to the poor geography and climate conditions, much work still needs to be done. Early Paleozoic tectonic melanges, including gabbros, diorites, basalts, andesites and cherts were reported in Qimantag Group of Yaziquan area, west of the Qimantag Mountain. We carry out study to intermediate-basic igneous rocks and cherts, including mineral chemistry, geochemistry and LA-ICP-MS U-Pb zircon dating.Intermediate-basic igneous rocks from Yaziquan generally experienced greenschist facies metamorphism. The mineral chemistry features show that amphiboles are mainly magnesium-hornblendes, while plagioclases are mainly albites.The geochemistry and isotope characteristics of intermediate-basic igneous rocks are as follows:(1) Hornblend gabbros are alkali gabbros, with low SiO2content (45.75%-50.54%), Al2O3content between12.82%and13.70%, high Mg#value (33.86-58.91), TiO2between2.24%and4.03%, CaO from6.62%to8.92%;ω(K20+Na20) is4.22%-5.44%, the average value is4.80%; Basalts are calc-alkali rocks. SiO2is from48.74%to49.95%, while A12O3content is between13.09%and14.18%; Mg#value is low (34.12-39.22), TiO2between2.58%and2.96%, CaO from7.76%to7.83%,ω(K20+Na20) values between4.25%and4.48%; diorites are mainly calc-alkali diorites. SiO2content is from54.04%to59.55%; Al2O3is ranging from14.54%to15.78%; all the samples in this group have high Mg#values (34.12-59.91); TiO2is between0.69%and1.67%, CaO mainly between4.15%and4.79%; ω(K20+Na20) values are from4.65%to8.01%; and andesites are mainly alkali rocks, with high SiO2contents (51.59%-59.59%); Al2O3is15.81%-18.01%; Mg#values are the highest one of the four groups (44.28-57.34); TiO2is from0.36%to1.43%; CaO ranging from4.40%to7.69%; ω(K20+Na20) is6.23%-7.79%;(2) REE patterns show that both basic and intermediate igneous rocks are enriched in LREEs, with (La/Yb)N ratios ranging from1.44to11.69;(3) Intermediate-basic igneous rocks are enriched in LILEs (like Ba, Th, U) and depleted in HFSEs (P, Zr), while Nb and Ta show negative anomaly;(4) εNd(t=480Ma) is from+1.1to+7.9, showing that the rocks were contaminated with subducted crustal materials. The characteristics above show that these intermediate-basic igneous rocks formed in an island arc setting.The cherts mainly developed within or above volcanic lavas. And they are of unique geochemical features.(1) SiO2content is relatively low, mostly ranging from68.77%to79.58%, while A12O3, Na2O and K2O contents are relatively high, with A12O3contents ranging from10.27%to14.67%, ω(SiO2)/ω(Al203+Na20+K20) ratios ranging from2.9to4.7. Al2O3/TiO2ratios are relatively high, ranging from40.75to79.31, while the ratios of MnO/TiO2range from0.11to0.27, which is similar to those of mud cherts formed in continental marginal island arc. The ratios of Al/(Al+Fe+Mn) range from0.62to0.76, indicating that the source materials are mainly derived of debris, in relative with biological causes.(2) REE content is relatively high, ranging from418.7×10-6to551.7×10-6. And∑LREE/∑REE values generally ranges from0.73to0.86, showing relative enrichment of LREE. NASC and chondrite normalized REE patterns demonstrate "V" shape without Ce anomaly. The ratios of LaN/CeN range from0.89to1.15, which is similar to those formed in continental margin.(3) The trace elements show enrichment of Hf, U and Th, while Cr, Ni and Sc shows negative anomaly, indicating that the cherts were not only influenced by continental materials but also volcanism. The correlation diagrams and the characteristics of major, trace and REE elements also indicate that the cherts are formed in continental marginal island arc.Zircon U-Pb dating for a diorite sample yields ages of480±3Ma, which represent the rock forming age. It means that the Qimantag oceanic basin already existed in early Ordovician and began subducting then, leading to the formation of island arc igneous rocks in Yaziquan. Zircons from pebbly sandstone are of magmatic features, with poor degree of rounding. Zircon U-Pb dating ages are550Ma-450Ma. Weighted mean ages of21zircons are479±4Ma, indicating that its source area undergoes an early Ordovician magmatic event, which is consistent with the diorite dating result. The discussion above shows that the intermediate-basic igneous rocks may be the origin of pebbly sandstone.