Source of Tinny Granite in Gejiu Tin Ore Deposit in Yunnan Province, China

Li, Ying Shu; Cai, Yanjun; Chen, Nan; Chen, Jiao Jiao; Wang, Lun; Zhang, Yi Ke; He, Da Qing

doi:10.4028/www.scientific.net/amr.634-638.3493

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“…The Gejiu alkaline intrusions as alkaline complex occurred with gabbro and granitic intrusions in the area, mainly consisting of silica‐saturated alkali syenites and silica‐undersaturated feldspathoid syenites. Previous studies have mostly focussed on the granitic rocks in Gejiu due to their close relationship to polymetallic tin mineralization (Cheng & Mao, 2010; Li et al, 2013). There are only a few studies on the geochemistry and chronology of the alkaline intrusions (Cheng, Mao, Chen, et al, 2008; Cheng, Mao, & Spandler, 2013; Huang et al, 2016, 2018; Li, Wang, & Zhang, 2018; Wang et al, 2019; Yu, 1993; Zhang, 2013).…”

Section: Introductionmentioning

confidence: 99%

Alkaline magmatism on Neo‐Tethyan extensional domains: Evidences from the Gejiu complex in Yunnan, China

et al. 2021

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The Gejiu alkaline complex (GAC) within the western part of the Youjiang Basin provides a window to investigate the evolution in the junction of Cathaysia, Yangtze and Indochina blocks. Here, we investigate the GAC in terms of their petrology, zircon U–Pb geochronology, whole‐rock geochemistry, and Sr–Nd isotopic data to gain insights into the origin and evolution of the alkaline magma. The GAC is lithologically composed of alkali syenites and feldspathoid syenites, in which some were altered into sericite syenites. Zircon U–Pb dating of the alkali syenites yielded an age of 85.03 ± 0.47 Ma, which is slightly older than the feldspathoid syenites. The alkali syenites and feldspathoid syenites are silica‐saturated and silica‐undersaturated, respectively, and are characterized by high alkalinity with K2O + Na2O of 11.55–17.08 wt% and Al2O3 of 18.57–22.49 wt%, low MgO of 0.11–1.39 wt%, weakly negative Eu anomalies, enrichments of LILEs such as Th and U, HFSEs like Zr and Hf but depletion of Ba, Sr, Nb, Ta, P, Ti, strongly fractionated LREEs to HREEs. Their uniform Sr–Nd isotope composition with initial 87Sr/86Sr = 0.708802–0.710571 and εNd(t) = −7.1 to −6.6 indicates that they were products of a homologous magma. They crystallized at c. 810–956°C and have a relatively high magmatic oxygen fugacity. Our geochemical and isotopic data proved that the GAC magma was derived from the low‐degree partial melting (<10%) of a phlogopite‐bearing‐enriched mantle that was metasomatized by subducting sediments and originated possibly from the spinel and garnet transition zone at a depth of 60–80 km and a pressure of about 1.8–2.4 Gpa. The primary magma experienced protracted two‐stage crystal fractionation of clinopyroxene+amphibole and biotite+K‐feldspar, forming alkali syenites and feldspathoid syenites. Crustal contamination plays a negligible role in their formation. Considering previous tectonic studies, it was therefore proposed that the GAC formed in an extensional tectonic setting related to the Neo‐Tethyan tectonic regions during the Late Cretaceous.

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Section: Introductionmentioning

confidence: 99%