| 其他摘要 | The region of Northern Xinjiang occupies a key part of the gigantic Central Asian Orogenic Belt, is characterized by special tectonomagmatic activity and metallogenesis, and has been attracting much attention from many Chinese and foreign geologists since early 1980’s. On each side of Junggar Basin in Northern Xinjiang, immense volume of A-type granites were emplaced. These granites and related deposits are the important objects of the geologists. Although there are many reported results, some key issues about the A-type granites and associated deposits, particularly their ages and genesis remain controversial. On the other hand, tin mineralization commonly connects to evolved peraluminous S-type granites. However, some medium-small tin deposits occur within or near the contact zones of A-type granitic intrusions in Karamaili tectonic belt, East Junggar. Thus, the relationship between the A-type granites and tin mineralization must be studied.
In this dissertation, the A-type granites from three tectonic belts around Junggar Basin (including Karamaili and Wulunguhe from East Junggar and Darabut from West Junggar) are selected as objects. The geochronological and geochemical studies have been carried out for some A-type granitic plutons in these belts. Then the petrogenesis of the A-type granites and their tectonic and metallogenic implications are discussed. Several conclusions drawn from this study are as follows:
(1) Zircon U-Pb dating yielded an age of 302~310Ma for the emplacement of aluminous and alkaline A-type granites in Karamaili, 305~320Ma for alkaline A-type granites and 270~280Ma for aluminous A-type granites in Wulunguhe, and 295~305Ma for aluminous A-type granites in Darabut from West Junggar. These isotopic ages with high precision further corroborate that the A-type granites from Junggar were formed in a post-collisional, extensional environment.
(2) The alkaline granites and alkali feldspar granites from three tectonic belts share all the mineralogical and geochemical features commonly observed in typical A-type granites. For major elements, they have high contents of SiO2 and alkalis but low abundances of Al2O3, CaO, MgO. With respect to trace elements, they are characterized by significant enrichment of some LILE (large ion lithophile elements, such as Rb, K, Th), HFSE (high field strength elements, such as Zr, Hf), REE (rare earth elements), and strong depletion in Ba, Sr, Eu. Based on their geochemical compositions, the Sujiquan biotite granites are actually divided into typical aluminous A-type granites, but not S-type granites proposed by others.
(3) The A-type granites from all three belts have relatively high positive Nd(T) values and two-stage Nd model ages older than their crystallization ages. However, their geological characteristics and geochemical compositions can’t be explained by the model of extensive fractionation from mantle-derived mafic magmas. These A-type rocks were most likely formed by fractional crystallization of granodioritic magmas, which were probably produced by partial melting of remnant basaltic oceanic crust with the same isotopic compositions as depleted mantle plus a little continental material.
(4) The development of early Permian A-type granites in East Junggar exemplified by Bieliartun pluton demonstrates that the post-collisional granitic plutonism in this area lasted a long period (about 60Ma). Moreover, the geochemical differences between the early Permian A-type granites and the late Carboniferous A-type granites suggest that the vertical growth of continental crust occurred in East Junggar or even Northern Xinjiang during the early Permian.
(5) The molybdenite samples from Sareshike tin deposit in East Junggar give a Re-Os isochron age of 307Ma, well consistent with the zircon U-Pb age of 306Ma for its wall rock (Sabei alkaline granites). In addition, low Re contents in the molybdenite suggest that ore-forming material probably originated from crustal sources. The temporal and original evidence argues that the A-type granites have a closely genetic connection with the tin deposit. |
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