Petrogenesis of syn-orogenic rare metal pegmatites in the Chinese Altai: Evidences from geology, mineralogy, zircon U-Pb age and Hf isotope

摘要

Recently, an increasing number of anatetic pegmatites have been confirmed. In contrast to the granitic pegmatite, the anatetic pegmatite has no relation to granite. It is therefore necessary to reacquaint the origin and classification of pegmatitic rocks. As one of the largest pegmatite provinces, the Chinese Altai provides a natural laboratory for pegmatite study. To date, the type, age, source and tectonic setting of the syn-orogenic pegmatites in the Chinese Altai remain unclear. In this work, the CMS (Chemical composition-Mineral assemblage-Structural geology) classification is primarily applied on the pegmatitic rocks in the Chinese Altai. Five types' pegmatitic rocks, including metapegmatite, pegmatoid, pegmatite, pseudopegmatite and granite pegmatite, are identified by their CMS features. In addition, the geology, mineralogy, zircon U-Pb ages and Hf isotopes of six pegmatites are presented in this work. The pegmatites, numbered JMHB02, QBL02, TLT01, AUK01, AMLG01 and TMLT01, are dated at 394.8 +/- 4.0 Ma, 402.6 +/- 5.5 Ma, 385.9 +/- 3.5 Ma, 368.0 +/- 4.0 Ma, 358.3 +/- 4.6 Ma and 333.0 +/- 6.0 Ma, respectively, indicating that they were formed during the syn-orogenic stage of the Altai orogenic process. They are exposed in the Qiongkuer domain in the southern Chinese Altai, juxtaposed with the late- and/or post-orogenic pegmatites in the same pegmatite field. No parental granite can be confirmed for all these pegmatites. Comparatively, the synorogenic pegmatites are scarce, and have lower mineralization grades than the late- and postorogenic pegmatites. Zircon Hf isotopic compositions of the syn-orogenic pegmatites have positive epsilon(Hf)(t) values of +2.06+8.28 and old T-DM2 ages of 1,234-860 Ma and are consistent with those of most of the syn-orogenic I- and S-granites, which indicates that the pegmatites and granites have a common immature crustal source from the early Paleozoic metasedimentary rocks. Compared to the post-orogenic pegmatites in the Central Altai domain, the syn-orogenic pegmatites in this work have larger positive epsilon(Hf)(t) values and younger T-DM2 ages, indicating that more juvenile components were involved in their magma sources. Combining this study with previous works, we suggest that the anatexis of metasediments induced by decompression and injection of heat energy and mantle-derived materials under a forearc extensional setting led to the formation of the syn-orogenic pegmatites in the Chinese Altai.