Alkaline magmatism on Neo‐Tethyan extensional domains: Evidences from the Gejiu complex in Yunnan, China. (27th May 2021)

• Record Type: Journal Article
• Authors: Wang, Yu‐Fei; Dong, Guo‐Chen; Santosh, M.; Liu, Chang; Chen, Wei; Liang, Jing‐Li; Zhang, Yin‐Chu
• Is Part Of: Geological journal. Volume 56:Number 8(2021)
• Page Start: 4331
• Page End: 4348
• Publication Date: 2021-05-27
• Subjects: Gejiu alkaline complex -- geochemistry -- magma source -- petrogenesis -- western Cathaysia block -- zircon U–Pb geochronology; Geology -- Periodicals; 551
• DOI: 10.1002/gj.4165
• Languages: English
• ISSNs: 0072-1050

Abstract

Abstract : 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 K2 O + Na2 O of 11.55–17.08 wt% and Al2 O3 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 87 Sr/ 86 Sr = 0.708802–0.710571 and ε Nd ( 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. Abstract : It is interesting that alkaline magmatism, consisting of syenites and feldspathoid syenites, took place at about 85 Ma by <10% partial melting of the lherzolite in enriched mantle source at depth of 60–80 km below the surface with condition about 810–956°C and 1.8–2.4 Gpa under an extensional tectonic setting.