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Early‐Cretaceous Syenites and Granites in the Northeastern Tengchong Block, SW China: Petrogenesis and Tectonic Implications
Author(s) -
ZHU Renzhi,
LAI Shaocong,
QIN Jiangfeng,
ZHAO Shaowei
Publication year - 2018
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.13631
Subject(s) - petrogenesis , zircon , geochemistry , geology , lithophile , rare earth element , lile , fractional crystallization (geology) , plagioclase , partial melting , magma , mantle (geology) , mineralogy , rare earth , quartz , volcano , paleontology
Whole‐rock major and trace element and Sr‐Nd isotopic data, together with zircon LA ICP‐MS in‐situ U‐Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U‐Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO 2 content (62.01–63.03 wt%) and notably high Na 2 O content (7.04–7.24 wt%) and Na 2 O/K 2 O ratios (2.02–2.10), low MgO, Fe 2 O 3 T and TiO 2 , enrichment of LILEs(large‐ion lithophile element) such as Rb, Th, U, K, and Pb) and obvious depletion HFSE(high field strength element; e.g. Nb, Ta, P, and Ti) with clearly negative Eu anomalies ( d Eu = 0.53–0.56). They also display significant negative whole‐rock ε Nd( t ) values of –6.8 and zircon ε Hf( t ) values(–9.11 to –0.27, but one is +5.30) and high initial 87 Sr/ 86 Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. 115.3Ma syenites were possibly derived from a sodium‐rich continental crustal source, and the fractionation of some ferro‐magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO 2 content (71.35–74.47 wt%), metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and Al 2 O 3 /(MgO+FeOT+TiO 2 ) ratios and moderate (Al 2 O 3 +MgO+FeOT+TiO 2 ) content. They show low initial 87 Sr/ 86 Sr (0.703408 to 0.704241) and ε Nd( t ) values (–3.8 to –3.5), plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong‐Nujiang Meso‐Tethys.