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

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 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.

Bobtraillite from Gejiu hyperagpaitic nepheline syenite, southwestern China: new occurrence and crystal structure

“…The increasing activity of water can also be substantiated by the replacement of primary anhydrous moxuanxueite by late-formed hydrous bobtraillite and catapleiite. This process could occur at a relatively low temperature (≤ 348 • C) (Chakrabarty et al, 2016), which is significantly lower than the magma crystallization temperature of Gejiu feldspathoid syenites (898 • C) (Wang et al, 2021).…”

“…The studied material is found in the feldspathoid (nephelinesodalite) syenite. The studied material was found in hand specimen 18CL18 collected by Wang et al (2021) near the location for their sample 18CL07. Wang et al (2021) gave a detailed account of the geology of the Changlinggang intrusions.…”

“…Furthermore, Li may be a more abundant constituent in hyperagpaitic rocks than is commonly realized. The Gejiu nepheline-sodalite syenite contains 107 ppm Li 2 O (Wang et al, 2021), and several lithium minerals have been found associated with the hyperagpaitic intrusions at Mont Saint-Hilaire (Van Velthuizen and Chao, 1989;Khomyakov et al, 1990;Chao and Ercit, 1991;McDonald et al, 2013), where the mineral assemblages resemble those in the Gejiu alkaline intrusive complex. The possibility of Li enrichment in hyperagpaitic intrusive complexes reminds us that cation-deficient minerals in such rocks, which have only been analyzed by the electron microprobe, need to be checked for Li, Be, and B, constituents that either cannot be analyzed with the electron microprobe or analyzed only with great difficulty.…”

Calcioancylite-(La), (La,Ca)2(CO3)2(OH,H2O)2, a new member of the ancylite group from Gejiu nepheline syenite, Yunnan Province, China

“…The intrusion forms an irregular ellipse body that is generally elongated in the S–N direction, covering ~28 km 2 . The Gejiu alkaline complex yielded emplacement ages of 80–82 Ma and intruded into Triassic sandstone and carboniferous rocks (Zhang, 2013; Huang et al , 2018; Wang et al , 2021). The pinkish feldspathoid syenite is a massive, medium- to coarse-grained rock and contains the feldspathoid minerals nepheline and sodalite as well as dark alkaline minerals.…”