Studies On Mesozoic A-type Granitoids And Their Rock Xenoliths In Lujiang-Zongyang Area, Anhui Province

Lujiang-Zongyang area in Anhui Province is located in the Lower reaches of the Yangtze River Fault-Depression Zone. Intensive magmatic activities and frequent crustal interaction with mantle taking place during J3-K1 are responsible for the formation of some sub-depression basins and uplifts and occurrence of a belt of A-type granitoids with various xenoliths in this area. A detailed petrographical, mineralogical and petrochemical study has been carried out on the A-type granitoids and their xenoliths in this thesis.The following are major achievements obtained in this thesis:1. Lithologies of the Mesozoic A-type granitoids in the Lujiang-Zongyang area mainly include pyroxene monzonite, quartz syenitic porphyry, syenite porphyry and alkali feldspar granite. They are rich in alkali, REE and LILE (e.g., Rb, Th and K), but poor in HFSE (e.g., Nb, Ta, Ti and Zr), Sr and P, and have higher ratios of 104*Ga to Al. In contrast, the xenoliths mainly consist of gabbro, pyroxene diorite and albite diorite. These rocks are poor in SiO2 and alkali, but rich in Cr, Co, Sc, V, Zr, and have higher ratios of Eu to Eu*.2. The A-type granitoids and xenoliths are similar in assemblage of major minerals. Salic mineral is dominated by feldspar, and mafic mineral by biotite and clinopyroxene. Feldspar includes plagioclase with oligoclase to andesine composition and alkali-feldspar with orthoclase to Na-orthoclase composition. Biotite is magnesian biotite, and clinopyroxene includes diopside and augite. The hosts contain more alkali-feldspar and biotite and less plagioclase and clinopyroxene. In contrast with this, the xenoliths contain less alkali-feldspar and biotite and more plagioclase and clinopyroxene.3. The petrographic, mineralogical and geochemical data presented in this thesis support the formation of the A-type granitoids through slight crustal material contamination combined with intensive fractional crystallization of basaltic magma derived from an enriched lithospheric mantle.4. The lithological association and geochemical signature of the A-type granitoids are consistent with their formation at the end of post-collisional magmatic activity in the setting of lithospheric extension. Therefore, their occurrence marks the transition from post-collisional to postorogenic tectonic setting and the end of the orogenic movement.5. The Sr and Nd isotopic data indicate enriched isotopic characteristics of the Mesozoic lithospheric mantle along the Yangtze River, Auhui province, and provide some evidence for the depth increasing of magmatic source during post-collisional to postorogenic period.