Petrogenesis of Early Cretaceous intrusions in the Sulu ultrahigh-pressure orogenic belt, east China and their relationship to lithospheric thinning
Resource
Chemical Geology 222,200-231
Journal
Chemical Geology 222
Pages
200-231
Date Issued
2005
Date
2005
Author(s)
DOI
246246/2006111501254446
Abstract
Geochronological, geochemical, whole-rock Sr–Nd & zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis & tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon 206Pb/ 238U age of 127F2 Ma & an 40Ar/ 39Ar age on hornblende of 123.5F0.4 Ma. A mafic enclave from this pluton has a SHRIMP zircon 206Pb/ 238U age of 124F3 Ma & a hornblende 40Ar/ 39Ar age of 124.2F0.4 Ma, indicating coeval crystallization of the mafic enclaves & host monzogranite. Whole rock 40Ar/ 39Ar dating gives an emplacement age of 111.2F0.1 Ma for the mafic dikes. The monzogranites have low MgO & Cr, high Na2O, & Sr–Nd–Hf isotopic data (87Sr / 86SrN0.7085, eNd(t)=20.5 & eHf(t)=22.5 to 56.6) consistent with derivation from Late Archean to Paleoproterozoic lower crust with involvement of mantle materials. The presence of coeval mafic enclaves with high eNd(t) & eHf(t) values indicates magma mixing & involvement of mantle-derived materials in the generation of the Shijiusuo pluton. The mafic dikes that intrude the monzogranite have characteristics of ultrapotassic rocks. Their geochemical features, such as high MgO (Mg# up to 75) & Cr (up to 1233 ppm), low TiO2 (1.11–1.24 wt.%) & total Fe2O3 (8.33–9.09 wt.%), enrichment in LILEs (e.g., Rb, Ba, Sr) & LREEs, & depletion in HFSE (e.g., Nb, Ta & Ti), together with initial 87Sr / 86Sr ratios of 0.7076–0.7078 & negative eNd(t) values (17.6 to 18.2), indicate they were derived from an amphibole-bearing, refractory lithospheric mantle. The Shichang–Fangzi monzodioritic to monzonitic rocks have zircon SHRIMP U–Pb ages of ~122 Ma. These rocks have Sr, Nd & Hf isotopic compositions (initial 87Sr / 86Sr=0.7083–0.7088, eNd(t)=16.5 to 17.7 & eHf(t)=20.4) similar to the mafic dikes in the nearby Shijiusuo pluton, indicating they were derived from a common source. High Rb & HREE, low Sr & Ba & strongly negative Eu anomalies indicate that the monzodioritic magmas resulted from pyroxene-and plagioclase-dominated fractionation of magma derived from an enriched mantle source. Taken together, these features indicate that Early Cretaceous magmatism in the Sulu orogenic belt was not related to Late Triassic
subduction or exhumation of the ultrahigh-pressure metamorphic rocks that characterize the Sulu region; instead they resulted
from mantle–crust interaction in an extensional setting, most likely induced by widespread removal of lithospheric mantle in the
eastern North China Craton during the Early Cretaceous.
Subjects
Early Cretaceous
Granitoid
Petrogenesis
Lithosphere removal
Sulu UHP belt
Type
journal article