Geochronology, Petrology And Geochemistry Of The Post-collisional Granites Along The Mayinebo Fault Zone, Altay, Xinjiang

The Altay orogenic belt, an important part of the Central Asia Orogenic Blet (CAOB), is famous for it voluminous granites, pegmatite and muscovite and rare mental deposits. The granites in Altay area include complicated rock types and have been well studied. However, only limited data have been reported on the post-orogenic granites in Altay. In this thesis, the geochronologic, petrologic and geochemical data of the post-orogenic granites along the Mayinebo fault zone (MFZ) in the eastern Altay are presented. They will provide a key constraints on the tectonic evolution and minerlization in Altay.Based on the geological mapping, field observation, petrologic and geochemical stuidies, Rb-Sr isochron and SHRIMP U-Pb zircon dating were carried out for five plutons along the MFZ in order to place constraints on the post orogenic tectonic evolution. The Wutubulake pluton at the south of MFZ is mainly composed of granodiorite and monzonitic granite with minor diorite distributed along the margin of the pluton. Whole-rock Rb-Sr isochron dating on it yields an age of 334±6Ma, whereas the whole-rock Rb-Sr isochron dating on the Areletuobie pluton (mainly composed of monzonitic granite) at the south of MFZ is obtained an age of 300±8.7Ma. The Hadeson pluton, outcropping to the south of Mayinebo falut zone consists of monzonite, quartz monzonite, gabbro, diorite monzonitic granite and K-feldspar granite. A zircon SHRIMP U-Pb dating from the monzonite gives the best estimate a crystallized age of 289.5±3.6Ma, which is consistant with a Rb-Sr isochron age of 281±14Ma, determined by whole rocks and minerals. Two representative plutons at the north of MFZ are chosen for the dating. The first one is Mayinebo pluton, including diorite, granodiorite and monzonitic granite, all of which appear gneiss-like structure, and contain some typical Al-riched minerals such as cordierite, tourmaline and sillimanite. Some rocks contain biotite up to 15% in volume. SHRIMP U-Pb zircon dating gives an average age of 283±4 Ma. The other one, Chaergan pluton, consists of diorite, granodiorite and monzonitic granite with minor gabbro. SHRIMP U-Pb zircon age yields the best crystallized age of 277.2±3.2Ma. In summary, the dating results suggest that the granites along MFZ mainly formed in post-collisional episodes if we consider the regional tectonic evolution.Major and trace element compositions show that the granites along MFZ are of high K calc-alkaline affinities. They are characterized by enrichment in LILE and LREE. Nevertheless, these granites exhibit distinct geochemical characteristics. For examples, the Wutubulake, Hadeson and Aretuobie plutons at the south of MFZ display the similar evolution trend, i.e., from calc-alkaline to alkaline, and their rock types are I-A types. They have relatively low initial Sr isotopic ratios and positiveε_Nd(t) values and juvenile Nd model ages. All geochemical characteristics imply that they are likely generated by partial melting of a juvenile mafic lower crust and variable basaltic materials derived from the mantle. The granites at the north of MFZ, the Mayinebo and Chaergan plutons, are also of high K cal-alkaline affinities. On the diagram of I-S classification, they belong to S type and I-S type, respectively. The Mayinebo pluton has relatively low SiO₂, CaO, Ba, Sr, Nb, P₂O₅, with Al₂O₃/TiO₂ ratio, and relatively high A/CNK values, MgO, total FeO(T), Cr, Ni, TiO2, REE(∑REE=126.8～218.11×10-6) concentrations, they are enriched in LREE with (La/Yb)_N=5-11,δEu=0.52-0.74. These features are consistent with those of the high-temperature strong-aluminous granites. It has negativeε_Nd(t) values (-0.3 to -3.5)and relatively older Nd model ages(1.2～2.2 Ga). Thus,it can be inferred that it was mainly derived from partial melting of the old crust with variable amounts of magmas derived from mantle.In combination with the previous studies on regional geology, it can be concluded that the rock type of the post-orogenic granites along the MFZ include I, A, S and I-S types. Those studies expand the rock types of the post-collisional granites in the Altay area. Moreover, the definition of the Mayinebo high-temperature strong-aluminous granites, provides a new type of the post-collisional granites in the Altay area and even in CAOB.