Petrogenesis And Deep Geological Processes Of The Late Mesozoic Magmatic Rocks In The Middle-Lower Yangtze River Belt

The Middle-Lower Yangtze River Belt (MLYRB) is located in central eastern China. It is one of the most important polymetallic metallogenic belt in China and is also an important magmatic activity belt of Late Mesozoic in eastern China. Late Mesozoic magmatic rocks are widely distributed in this region. Although a series of studies have been carried out on these Late Mesozoic magmatic rocks, there are still controversies in petrogenesis and correspondingly deep geodynamic processes. Comparing with a study of single-stage magmatism, a comprehensive and comparative study on magmatic rocks which are belong to different stages and different rock types in the same tectonic unit can give more detailed information about the petrogenesis and deep geological processes. This paper carried out integrated studies of whole-rock major and trace elements, Sr-Nd isotopes, zircon geochronology, and in-situ zircon Hf-0 isotopes on intrusive rocks (Xiaotongguanshan intrusion, Xiaoqingtang intrusion, Shatanjiao intrusion in Tongling area and Binjiang intrusion, Banshiling intrusion, Fushan intrusion in Fanchang area) and volcanic rocks (Longwangshan Formation, Dawangshan Formation, Gushan Formation, Niangniangshan Formation in Ningwu basin; Zhongfencun Formation, Chisha Formation, Kedoushan Formation in Fanchang basin) to trace the magma sources and deep evolution processes.The results of zircon U-Pb dating showed that the forming ages of Xiaotongguanshan intrusion and its inclusions, Xiaoqingtang intrusion and Shatanjiao intrusion in Tongling area were 140-135 Ma; the forming ages of Binjiang intrusion, Banshiling intrusion and Fushan intrusion in Fanchang area were 127-125 Ma; the erupting ages of volcanic rocks in Ningwu and Fanchang basins were concentrated in 132～129 Ma. The late Mesozoic magmatism in the MLYRB (precluding Ningzhen area) can be subdivided into three stages:high-K calc-alkaline intrusions (148-133 Ma), shoshonitic volcanic rocks and subvolcanic rocks (133-127 Ma), and A-type granitoids (127-123 Ma). Xiaotongguanshan, Xiaoqingtang, and Shatanjiao intrusions in Tongling area belonged to the first stage of magmatism; volcanic rocks in Ningwu and Fanchang basins belonged to the second stage of magmatism; Binjiang, Banshiling, and Fushan intrusions in Fanchang area belonged to the third stage of magmatism.The rock types of Xiaotongguanshan intrusion and its enclaves, Xiaoqingtang intrusion, and Shatanjiao intrusion in Tongling area were sequentially quartz monzodiorite,diorite,granodiorite-porphyry and granodiorite,belonging to high potassium calc-alkaline series. The zircon εHf(t) values and δ18O values of the intrusions were ranging from -45.4 to -6.5 and from 5.7‰ to 10.2‰. The zircon εHf(t) values and δ18O values of the dark enclaves were ranging from -38.6 to -6.6 and from 5.7‰ to 10.1‰. Accordingly, Sr-Nd isotopes of the Xiaoqingtang and Shatanjiao intrusive rocks also were enriched (87Sr/86Sr(t)=0.7081～0.7094, εNd(t)=-13.5～-8.3). The chemical variations of these intrusive rocks can be attributed to partial melting in different degrees and mixing of magmas which were derived from a mantle and a crust. Subordinate fractional crystallization (FC) also occurred during evolution processes of these magmas, but only occurred in the range of 68% to 72% of SiO2 contents. The basic enclaves in Tongling area were formed by mixing of magmas, main of which were derived from an enriched mantle that had been metasomatized by fluid/melts from a subducting oceanic slab, and subordinationr of which were derived from the Archean lower continental crust (LCC). A small quantity of Neoproterozoic crustal materials might be assimilated into the basic magmas as they ascended to shallow crustal level. A little earlier, or almost simultaneously, the basaltic magmas derived from the same enriched mantle source underplated into the mantle-crust boundary and were mixed with magmas derived from the ancient LCC to form a juvenile thickened lower crust. Partial melting of the juvenile thickened lower crust in the pressure of garnet phase formed the intermediate intrusive rocks. Some of the intermediate magmas underwent a process of FC (involving about 9% plagioclase+5% biotite+4% amphibole) to form the intermediate-acidic intrusive rocks.The rock types of Longwangshan, Dawangshan, Gushan, and Niangniangshan Formations in Ningwu basin were trachyandesite, trachyandesite, andesite and nosean phonolite, respectively. The Niangniangshan volcanic rocks belonged to shoshonite series and their 87Sr/86Sr(t) values and εNd(t) values were ranging from 0.7056 to 0.7063 and from -2.1 to -0.9. Zircon εHf(t) and δ18O values of the volcanic rocks in Ningwu basin were ranging from -13.3 to 3.8 and from 5.4‰ to 7.6‰. The rock types of Zhongfencun, Chisha, and Kedoushan Formation in Fanchang basin were rhyolite, trachyandesite and rhyolite, respectively. Their zircon εHf(t) and δ18O values were ranging from -8.2 to -2.2 and from 6.4‰ to 8.2‰. Crystallization differentiation was weak in the formation processes of the volcanic rocks in Ningwu basin. The volcanic rocks of Gushan and Niangniangshan Formations were derived from an enriched mantle that had been metasomatized by fluid/melts from a subducted oceanic slab and partial melting might play a major role in the formation processes of these volcanic rocks. The volcanic rocks of Longwangshan and Dawangshan Formations were also derived from the same enriched mantle, but the initial magmas were contaminated by Archean LCC components during their ascent. The volcanic rocks from Fanchang basin had similar magma sources. They were formed by mixing of magmas derived from the Mesoproterozoic crust and the enriched lithospheric mantle, experiencing crystallization differentiation in some degree.The Binjiang, Banshiling, and Fushan intrusive rocks in Fanchang area were medium grained granite, K-feldspar granite and quartz monzonite, respectively. The Binjiang and Fushan intrusions were A2-type granitic affinity, and the Banshiling intrusion was Ai-type granitic affinity, and they all belonged to the high potassium calc-alkaline series. Their 87Sr/86Sr(/) and em(t) values were ranging from 0.7072 to 0.7080 and from-8.3 to-6.6. Their zircon εHf(t) and δ18O values were ranging from-8.0 to 0 and from 6.7%o to 10.3%o. Crystallization differentiation in some degree existed in the forming processes of Binjiang and Fushan intrusions, but was relatively weak in the forminf processes of Banshiling intrusion. Binjiang and Fushan intrusive rocks were formed by reworking of the Mesoproterozoic juvenile crust that might have undergone previous extraction of granitic melts. Banshiling intrusive rocks were mainly derived from the metasomatized mantle and were contaminated by the Mesoproterozoic crust components during their ascent.Three stages of magmatisms in the MLYRB revealed a progressive evolution of magma P-T conditions and a transformation of the tectonic setting, and indicated the deep geological processes of crust-mantle interaction, including mantle metasomatism, basaltic magma underplating, partial melting, magma mixing and so on, in the background of low angle subduction of the paleo-Pacific plate, following by increasing of the subduction angle and roll-back of the subducted slab, corresponding for a transformation of tectonic setting from the continental active marginal arc to back-arc.