Petrogenesis And Geodynamic Processes Of The Paleozoic Magmatism In The Southwestern Of The Alxa Block

The Alxa Block, located in the southern CAOB, is usually considered as the western part of the NCC, and connects with the Tianshan-Beishan orogenic belt to the west and the Great Xinganling-Inner Mongolian orogenic belt to the east. Thus, the Alxa Block is the key region in understanding the tectonic evolution of the southern CAOB. Phanerozoic granitoids are widely exposed in the Alxa Block. The Paleozoic geodynamic setting of the Alxa Block and its tectonic connection with the other part of the CAOB are still confused. In the paper, author presents field geology, petrography, LA-ICP-MS zircon U-Pb geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic compositons for the Paleozoic intrusive rocks from the southwestern margin of the Alxa Block. The studying aims are (1) revealing the petrogenesis of these magmatic; and (2) discussing the Paleozoic tectonic evolution of the Alxa Block. The main results are as follows:1.16 intrusive rocks samples from the southwest of the Alxa Block were analysed for in situ zircon U-Pb dating. The magmatic activities in the southwest of the Alxa Block belt can be divided into three episodes:Devonian (423-397Ma), Late Carboniferous (320-315Ma), and Early Permian-Middle Permian (289-265 Ma).2. The Devonian granitoids are widely exposed in the southwest of the Alxa Block(Beidashan batholith, Yabulai batholith and Gaojiajiao area).All of the samples can be divided into two groups:high Sr/Y granites and low Sr/Y granites. The Devonian granites have SiO2 (69.21-74.60 wt.%) with metaluminous to weakly peraluminous. The Devonian granites show high Isr ratios (0.7074-0.7225) and negative whole-rock εNd(t) values (-12.1 to-2.6), with Nd two-stage depleted mantle model ages of 1.4-2.1 Ga. Zircon Hf isotopic data show εHf(t) values from-18.7 to 2.0 and TDMC=1.3-2.4 Ga,. These geochemical and Sr-Nd-Hf isotopic data imply that they were derived from partial melting of Paleoproterozoic-Mesoproterozoic crustal materials. For the high Sr/Y granites, we suggest that the Beidashan granites Group I resulted from fluid-absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of～12 kar; the Beidashan granites Group II formed by fluid-present partial melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of～10 kar; Both the Yabulai porphyritic granites and the Yabulai two-mica granites Group 2 formed by fluid-present partial melting of crustal materials. For the low Sr/Y granites, we suggest that the Yabulai two-mica granites Group 1 derived from partial melting of a source region dominated by subordinate metapelite, and Gaojiajiao porphyritic granites were derived from fluid-absent partial melting of a source region dominated by amphibolite.3. The Late Carboniferous gabbro-diorite and quartz diorite porphyrite are exposed in Beidashan batholith. The gabbro-diorite have relative low SiO2 (47.52-56.29 wt.%), MgO (3.86-4.89 wt.%), Cr (2.05-68.1 ppm) and Ni (0-36 ppm). They show slightly positive Pb anomaly and slightly negative Ti anomaly, suggesting that they partial melting of metasomatized mantle. The quartz diorite porphyrite have SiO2 (64.21-68.99 wt.%) and Al2O3 (14.38-16.60 wt.%) with A/CNK ratios of 0.96～ 1.01. All samples of the quartz diorite porphyrite exhibit high high Sr/Y (34-56) and (La/Yb)N (10.7-12.5) ratio with slightly negative or positive Eu anomaly (0.90-1.04), indicating geochemical features similar to adakites or high Sr/Y magmas. The Sr-Nd-Hf isotopic compositions (ISr=0.7070 to 0.7088, εNd(t)=-1.8, and εHf(t)= +5.5～+8.2) of quartz diorite porphyrite are comparable to those of the Late Carboniferous gabbro-diorite. Thus, they were most likely derived from the partial melting of new underplated basaltic rocks. We conclude that the Quagan Qulu back-arc basin southward in Late Carboniferous.4. The Early Permian-Middle Permian granitoids are widely exposed in the Yabulai batholith and Gaojiajiao area. The Yabulai porphyritic granites contain SiO2 contents of 70.36-74.16 wt.% with metaluminous(A/CNK=0.98-0.99). The Sr-Nd-Hf isotopic compositions (ISr=0.7081 to 0.7084, εNd(t)=-8.0 to-8.2, and εHf(t)=-11.1～-4.1) of these Yabulai porphyritic granites derived from remelting of an ancient basement. The Yabulai quartz diorite which share the same Sr-Nd-Hf were probably generated by mixing between crust- and mantle-derived magmas. The moyite have high SiO2 (SiO2>75 wt.%), K2O (34.16-5.25 wt.%) and K20+Na20 (8.04-9.13 wt.%) and relative low Al2O3(10.78-12.26 wt.%), CaO(0.37-0.89 wt.%) and TiO2(0.05-0.14 wt.%). The K-feldspar granites are characterized by significant negative Eu*/Eu anomalies (0.02-0.15). These geochemical characteristics indicate that the K-feldspar granites have an A-type affinity. The Hf isotopic data imply that they were derived from partial melting of Paleoproterozoic crustal materials. Combined with reaional studies, the K-feldspar granite formed in a post-collisional tectonic setting.5. The free water in the mid-to lower crust is most likely related to the Quagan Qulu back-arc extension in Early Paleozoic, thus we propose that the Devonian granites might witness the back-arc rifting of the Quagan Qulu back-arc basin. In Late Carboniferous, the Quagan Qulu back-arc basin southward and was closed in Middle Permian.6. The petrogenesis of high Sr/Y granitoids in orogens is multifarious. The high Sr/Y ratios do not directly correlate to deep melting. Source and magma generation machenism (e.g. pressure/depth, temperature and water) will also conttibute to high Sr/Y values in the melt.