No abstract

No abstract

No abstract

No abstract

No abstract

No abstract

Abstract:In the arc (basin)-back area of the Yidun arc belt in the north segment of the Sanjiang tectonic zone, southwestern China, there occurs a post-orogenic granite belt extending for more than 300 km in NNW direction. It strides across two different tectonic units of the arc (basin)-back area and the subduction area, and is accompanied by extensive Ag-Sn polymetallic mineralizations. More than ten granite bodies have very similar geochemical characteristics: high SiO 2 (73.8~76.3 wt%) and K 2 O+Na 2 O (7.16~8.41 %), and low Al 2 O 3 (11.9~13.6 %), CaO (0.46~1.54 %) and MgO (0.16~0.61 %), as well as high enrichment of Nb, Ta, Ga and Y, and strong depletion of Sr and Eu. Most of these features are peculiar to A-type granite. Rb-Sr and 40 Ar/ 39 Ar isotopic dating results indicate that the formation ages of the granites decrease from 103.7 Ma of the north end to 75.2 Ma near the south end, and that the magmatism became younger from north to south. The tectonic environment analysis clearly reveals that they were formed in post-orogenic within-plate extension settings. The magma genesis was controlled by a united crustal extension regime after the arc-continent collision. The granites have low ε Nd values ranging from -4.96 to -8.40, whereas the ε Sr values vary greatly ranging from -31.7 to 296, reflecting that the source composition transited from mantle -differentiated igneous rocks in the north to basement -dominated metamorphosed sedimentary rocks in the south. Under high temperature and water-absent conditions, the anatexes of the crustal rocks made a great amount of plagioclase separated from melts and left in magma sources. Through this mechanism, the post-orogenic granites took geochemical characteristics such as low Al 2 O 3 and CaO, and strong depletion of Sr and Eu.

No abstract