A-type granites derived from dehydration melting of calc-alkaline granitoids in East Junggaer (NW China): Implications for the origin of aluminous and peralkaline A-types

Huang，Chao1,2; Chen，Bin2; Sun，Keke3

2023-03-01

10.1016/j.lithos.2023.107043

LITHOS   Impact Factor & Quartile

0024-4937

1872-6143

A-type granites always co-exist with calc-alkaline granitoids in post-collisional settings, and were divided into aluminous A-type and peralkaline A-type on the base of their contrasting chemical compositions and mineralogy. The source characteristics and formation P-T conditions of the two categories of A-types should be very different. To unravel the origin of A-type granites and their genetic relationship with calc-alkaline granitoids, we report combined petrological, whole-rock major and trace elements, mineral chemical and Nd[sbnd]Sr isotopic data for the coexisting calc-alkaline granitoids and two suites of A-type granites (one aluminous, the other peralkaline) from East Junggaer (NW China) of the Central Asian Orogenic Belt. Our data indicate that the calc-alkaline granitoids were derived mainly from melting of the Paleozoic metabasite that were buried deep in the lower crust. Subsequent melting of the calc-alkaline granitoids (containing equally important biotite and hornblende), under relatively low pressures, fluid-absent and reducing conditions, yielded first a parent magma to the aluminous A-type granites through biotite dehydration melting, leaving a hornblende-bearing charnokitic solid residue. Re-melting of such a residue at even higher temperatures yielded a parental magma to the peralkaline A-type granites. Significant feldspar-dominated fractionation (∼10–30%) followed during the processes of magmatic evolution. This hypothesis is supported by the slightly younger intrusion age of peralkaline A-type (ca. 300.6 Ma) than aluminous A-type (ca. 302–304.5 Ma), the consistent Nd[sbnd]Sr isotopic compositions of the two A-types ((Sr/Sr) = 0.7040–0.7042, ε(t) = +5.8 ∼ +6.4) and the calc-alkaline granitoids ((Sr/Sr) = 0.7036–0.7045, ε(t) = +5.9 ∼ +6.3), the significantly lower Rb, Li, Cs contents and Rb/Zn, Li/Zn ratios in the peralkaline A-types than in the aluminous ones, and modeling study based on major and trace elements as well. Our data explain the co-existence of A-type granites with, and always being younger than, calc-alkaline granitoids in many orogenic belts. We suggest that source characteristics is a major factor controlling the formation of aluminous and peralkaline A-type granites, with biotite and hornblende as dominant ferromagnesian phase in the source calc-alkaline granitoids, respectively. Feldspar-dominated fractionation processes could have contributed to the formation of the geochemical features typical of A-type granites, e.g., the extremely low Sr (and Ba), Eu, CaO, MgO, and high Ga/Al ratios, and the enriched alkalis relative to alumina with high (NaO + KO)/AlO ratios. This study confirms the previous experimental work on dehydration melting of calc-alkaline granitoids, and further indicates that the compositional discrepancies between experimental melts and natural A-types could be ascribed to the lack of fractionation processes in the former.

A-type granites Calc-alkaline granitoids Dehydration melting

SCI

English

Corresponding

Stable Support Plan Program of Shenzhen Natural Science Fund["GXWD20201230110313001","20200925161932001"] ; National Natural Science Foundation of China[41530206]

Geochemistry & Geophysics ; Mineralogy

Geochemistry & Geophysics ; Mineralogy

WOS:000963471000001

ELSEVIER

GEOSCIENCES

2-s2.0-85147829601

Scopus

1.School of Resources and Environmental Engineering,Hefei University of Technology,Hefei,230009,China
2.Department of Earth and Space Sciences,Southern University of Science and Technology,Shenzhen,518055,China
3.School of Earth Sciences and Engineering,Hohai University,Nanjing,211100,China

Huang，Chao,Chen，Bin,Sun，Keke. A-type granites derived from dehydration melting of calc-alkaline granitoids in East Junggaer (NW China): Implications for the origin of aluminous and peralkaline A-types[J]. LITHOS,2023,440-441.

Huang，Chao,Chen，Bin,&Sun，Keke.(2023).A-type granites derived from dehydration melting of calc-alkaline granitoids in East Junggaer (NW China): Implications for the origin of aluminous and peralkaline A-types.LITHOS,440-441.

Huang，Chao,et al."A-type granites derived from dehydration melting of calc-alkaline granitoids in East Junggaer (NW China): Implications for the origin of aluminous and peralkaline A-types".LITHOS 440-441(2023).