Petrogenesis And Deep Processes Of Late Mesozoic A-type Grinate In The Lower Yangtze Region

The late Mesozoic A-type granites in the Lower Yangtze region is the weak link of studies about igneous rocks in this area. On the basis of previous studies and field observations, zircon U-Pb dating, major and trace element geochemistry, whole rock Sr-Nd isotopic and zircon Hf-O isotopic geochemistry analysis are carried out on the Guichi pluton from the Guichi region, the Binjiang, Fushan and Banshiling pluton in the Fanchang basin.Zircon U-Pb ages of igneous rocks are concentrated. The206Pb/238U weighted average ages of edge phase of Guichi pluton is127.7±1.8Ma,125.9±1.6Ma and124.6±2.0Ma, the center phase of Guichi pluton of123.0±3.7Ma, the coarse-grained phase of Binjiang pluton of124.6±4.7Ma, the fine-grained phase of Binjiang pluton of123.0±1.8Ma, the Banshiling pluton of124.9±1.7Ma, the Fushan pluton of126.4±1.7Ma. These plutons are simultaneously intruded with the A-type granite in the North belt of the Yangtze River and belong to the third stage of late Mesozoic igneous rocks in the Lower Yangtze region. Besides the age of this stage, some zircons of center phase of Guichi pluton also show age information of the first stage of late Mesozoic igneous rocks in the Lower Yangtze regions, which is145.4.0±3.2Ma. The two generation growth of zircon indicated that the pluton has undergone two magmatic activities. Besides, we also find an Archean zircon in fine-grained phase of Binjiang pluton, which could provde the evidence for the existence of the Archean basement in the Lower Yangtze regions.Guichi, Fushan,and Banshiling pluton is a typical A-type granite, and Binjiang pluton is highly fractionated granite, similar to the A-type granites.These intrusions are characterized by varying degrees of negative Eu anomaly as well as LREE-enrichment, with selective depleted characteristic which is the unique of highly fractionated granite, such as high field strength elements of Zr, Hf, Nb, and large ion lithophile elements of Rb, U Th enrichment and Ba, Sr depletion. These characteristics are significantly different from that of the early stage of Late Mesozoic igneous rocks in the Lower Yangtze region.The granites in Guichi region and Fanchang basin show different Sr-Nd isotopic features. Samples of Guichi pluton are characterized by scattered Initial87Sr/86Sr(t) ratios (0.7065-0.7111), and concentrated εNd(t) values(-6.7～-7.3). But the Sr-Nd isotopic compositions of granites from the Fanchang basin showed the opposite features. Guichi, Banshiling and Fushan pluton show approximately the same value of εNd(t)(-6.7～-7.8), which is consistent with the value of Shangxi Gr(calculation with125Ma), and can represents the source of the upper and middle crust. The δ180ratios of Banshiling pluton (6.7%o-7.4%o) is lightly lower than that of Fushan pluton (7.1‰～9.1‰). These means the source materials did not experience the surface exchange process and should be representative of the product of remelting or partial melting of deep materials. The O isotopic compositions of Binjiang pluton (8.0‰～10.3‰) are similar to that of the Neoproterozoic S-type granodiorite in the Southern Anhui. It implies a close contact on the material source. The average εHf(t) value of Banshiling pluton (-5.3) is close to that of Fushan pluton (-5.5), and the value of Binjiang pluton is-4.5. These are consistent with the value of Neoproterozoic S-type granodiorite in the Southern Anhui which is calculated accordance with the176Lu/177Hf ratio of average crust with age of125Ma. Comprehensive analysis shows that, the granites in Fanchang basin derived from the remelting of the middle and upper crust represented by the Neoproterozoic granites.The tectonic setting in the Lower Yangtze region transformed from extrusion to extension from148Ma, and the thermal disruption caused by the upwelling asthenosphere lead to the partial melting of the lithospheric mantle and lower crust as well as the extensive crust-mantle interaction. To130Ma, large amounts of materials from the lithospheric mantle erupted to the surface and formed the alkaline and igneous rocks with the increased intensity of the extension. During the126-122Ma period, the A-type granites derived from the remelting of the middle and upper crust indicated that the middle and upper crust affected by thermal disruption started to partial melt. And this also demonstrates that the increased uplifting of the magmatic source and the intensity of the extension. To126～122Ma, the peak extension was achieved.