RECOGNITION OF A 600-KM-LONG LATE TRIASSIC RARE METAL (Li-Rb-Be-Nb-Ta) PEGMATITE BELT IN THE WESTERN KUNLUN OROGENIC BELT, WESTERN CHINA

The rising demand of strategic metals, especially lithium, necessitates discovery of new resources to meet the global supply chain. Recently, several pegmatite-hosted rare metal (Li-Rb-Be-Nb-Ta) deposits have been discovered in the Western Kunlun orogenic belt, making it a new world-class rare metal resource (estimated ~7 Mt Li2O and 0.16 Mt BeO). Understanding the metallogenesis of this belt is critical to further evaluate the rare metal potential. In this study, columbite-tantalite (coltan) and monazite from rare metal pegmatites and zircon from potential parental granites were collected from five representative rare metal pegmatite deposits in the western, middle, and eastern parts of the Western Kunlun orogenic belt for U-Pb geochronology. The results indicate that despite the distances of the sampling localities in different parts of the Western Kunlun orogenic belt, the ages of pegmatite-hosted rare metal mineralization fall in a narrow range of ca. 208–204 Ma. These rare metal pegmatites are temporally and spatially related to adjacent postorogenic granites emplaced following the closure of the Paleo-Tethys Ocean. The compositional characteristics of K-feldspar, biotite, and muscovite of the granites and pegmatites, along with regional mineralogical and textural zonation of the pegmatites, suggest that the rare metal pegmatites were derived from the volumetrically much more important, highly fractionated granitic intrusions. We propose that, in combination with the data from previous studies, the 218–204 Ma interval represents a newly recognized rare metal metallogenic period linked with granitic intrusions in the Western Kunlun orogenic belt, revealing a 600-km-long late Triassic rare metal pegmatite belt composed of multiple ore fields formed in a similar metallogenic setting. These results emphasize the importance of identifying fertile, Late Triassic to Early Jurassic granitic intrusions for rare metal pegmatite exploration. Furthermore, combined with recent studies on the Songpan-Ganzi rare metal pegmatite belt along the eastern segment of the Paleo-Tethys, this study further highlights the great potential of rare metal resources in this global tectonic zone.

Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

The supra-subduction zone ophiolite or ophiolitic mélange formed in the forearc setting is generally considered to be a key geological record for subduction initiation (SI) with petrological characteristics comparable to the SI-related rock sequence from forearc basalt (FAB) to boninite in the Izu-Bonin-Mariana subduction zone. Nevertheless, the standard FAB and boninite are generally difficult to observe in the forearc rocks generated during SI. Yet, a typical rock sequence indicating the SI of the western Proto-Tethys Ocean is reported for the first time in the Qimanyute intra-oceanic forearc system in the western Kunlun Orogen, Northwest Tibetan Plateau. The magmatic compositions, which range from less to more high field strength element (HFSE)-depleted and large ion lithophile element (LILE)-enriched, are changing from oceanic plagiogranites (ca. 494 Ma) to forearc basalt-like gabbros (FAB-Gs, ca. 487 Ma), boninites, and subsequent Nb-enriched gabbros (NEGs, ca. 485 Ma), which are thus consistent with the Izu-Bonin-Mariana forearc rocks as well as the Troodos and Semail supra-subduction zone-type ophiolites. The geochemical data from the chemostratigraphic succession indicate a subduction initiation process from a depleted mid-oceanic-ridge (MORB)-type mantle source with no detectable subduction input to gradual increasing involvement of subduction-derived materials (fluid/melts and sediments). The new petrological, geochemical, and geochronological data, combined with the regional geology, indicate that the well-sustained FAB-like intrusive magmas with associated boninites could provide crucial evidence for SI and further reveal that the SI of the western Proto-Tethys Ocean occurred in the Late Cambrian (494−485 Ma).

Supplemental Material: Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Detailed petrological and geochemical data.

Supplemental Material: Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Detailed petrological and geochemical data.

Kinematics of Cenozoic shortening across the foothills of the Western Kunlun Range (Xinjiang, China): the case of the Hotan anticline

<p>Quantitative constraints on the Cenozoic deformation of the northwestern edge of the Tibetan Plateau remain limited, in particular in terms of shortening rates and of their possible evolution over time. This is indeed the case for the Western Kunlun Range, along the southwestern rim of the Tarim Basin, even though surface geological data and an extensive database of seismic profiles allow to explore the sedimentary record of Cenozoic deformation. Here, we take advantage of these data to document the structural geometry and Cenozoic kinematics of the large scale east-west striking Hotan anticline along the mountain front. Four balanced cross-sections are constructed, and the temporal evolution of deformation is deciphered from the exceptionally seismically well imaged growth strata on the forelimb of the anticline.</p><p>The fold results from a broad unfaulted basement ramp anticline, subsequently deformed by a duplex structure that developed in the footwall units. The total shortening of the Hotan thrust system is relatively constant along strike, from ~40 to ~32 km. The shortening accommodated by the duplex varies laterally from west to east, from ~50-40 % to 0 % of the total shortening. </p><p>Two distinct successive patterns of growth strata are recognised in the forelimb, and are interpreted to be representative of deformation on the basement ramp, followed by deformation related to the growth of the underlying duplex. Deformation on the basement ramp initiated by ~17 Ma, when calibrating growth seismic reflectors on surface magnetostratigraphic sections. Deformation of the underlying duplex began at ~12 Ma to the west and subsequently propagated eastward.</p><p>From these results on shortening and timing of deformation, we determine a shortening rate of 4-3 mm/yr from ~17 to ~7 Ma across the Hotan anticline. We find a significant subsequent decrease in shortening rates, possibly down to <0.5 mm/yr since the uppermost Miocene. These rates are compared to existing values and their regional significance is discussed.</p>