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Geochemical evidence for the production of granitoids through reworking of the juvenile mafic arc crust in the Gangdese Orogen, southern Tibet

Tang Yuwei, Chen Long, Zhao Zifu and Zheng Yongfei
Geochemical evidence for the production of granitoids through reworking of the juvenile mafic arc crust in the Gangdese Orogen, southern Tibet
Geological Society of America Bulletin (November 2019) 132 (7-8): 1347-1364

Abstract

Although continental crust is characterized by the widespread occurrence of granitoids, the causal relationship between continental crust growth and granitic magmatism still remains enigmatic. While fractional crystallization of basaltic magmas (with or without crustal contamination) and partial melting of mafic lower crust are two feasible mechanisms for the production of granitoids in continental arc regions, the problem has been encountered in discriminating between the two mechanisms by whole-rock geochemistry. This can be resolved by an integrated study of zircon U-Pb ages and Hf-O isotopes together with whole-rock major-trace elements and Sr-Nd-Pb isotopes, which is illustrated for Mesozoic granitoids from the Gangdese orogen in southern Tibet. The results provide geochemical evidence for prompt reworking of the juvenile mafic arc crust in the newly accreted continental margin. The target granitoids exhibit high contents of SiO (super 2) (65.76-70.75 wt%) and Na (super 2) O+K (super 2) O (6.38-8.15 wt%) but low contents of MgO (0.19-0.98 wt%), Fe (super 2) O (super 3) (0.88-3.13 wt%), CaO (2.00-3.82 wt%), Ni (<5.8 ppm), and Cr (< or =10 ppm). They are enriched in large ion lithophile elements, Pb, and light rare earth elements but depleted in high field strength elements. The granitoids are relatively depleted in whole-rock Sr-Nd isotope compositions with low ( (super 87) Sr/ (super 86) Sr) (sub i) ratios of 0.7043-0.7048 and positive epsilon (super Nd) (t) values of 0.5-2.6, and have relatively low (super 207) Pb/ (super 204) Pb and (super 208) Pb/ (super 204) Pb ratios at given (super 206) Pb/ (super 204) Pb ratios. Laser ablation-inductively coupled plasma-mass spectrometry and secondary ion mass spectrometry U-Pb dating on synmagmatic zircons yield ages of 77 + or - 2-81 + or - 1 Ma in the Late Cretaceous for their emplacement. Relict zircons have two groups of U-Pb ages in the late Mesozoic and the late Paleozoic, respectively. The whole-rock Sr-Nd isotopes in the granitoids are quite similar to those of Late Cretaceous mafic rocks in the Gangdese batholith. In addition, both synmagmatic zircons and relict zircons with Late Cretaceous U-Pb ages exhibit almost the same Hf-O isotope compositions to those of the slightly earlier mafic rocks. All these observations indicate that the granitoids were mainly derived from partial melting of the juvenile mafic arc crust. Therefore, reworking of the juvenile mafic arc crust is the mechanism for the origin of isotopically depleted granitoids in southern Tibet. It is this process that leads to differentiation of the juvenile mafic arc crust toward the felsic lithology in the continental arc. In this regard, the granitoids with depleted radiogenic isotope compositions do not necessarily contribute to the crustal growth at convergent plate boundaries.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 132
Serial Issue: 7-8
Title: Geochemical evidence for the production of granitoids through reworking of the juvenile mafic arc crust in the Gangdese Orogen, southern Tibet
Affiliation: University of Science and Technology of China, School of Earth and Space Sciences, Hefei, China
Pages: 1347-1364
Published: 20191107
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 165
Accession Number: 2020-006785
Categories: Igneous and metamorphic petrologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2020025
Illustration Description: illus. incl. 3 tables, geol. sketch maps
N29°03'00" - N29°13'60", E93°09'00" - E94°17'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 202006
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