Adakitic signature formed by fractional crystallization: An interpretation for the Neo-Proterozoic meta-plagiogranites of the NE Jiangxi ophiolitic mélange belt, South China

Abstract

The low-temperature/high-pressure metamorphic rocks which are exposed as exotic blocks within the serpentinite matrix of the NE Jiangxi ophiolitic mélange belt consist of enclave-bearing and -free meta-plagiogranites. Both rock types are characterized by a metamorphic mineral assemblage consisting of Ab-rich plagioclase + quartz +/− clinopyroxene + amphibole while rutile, titanite and zircon occur as accessories. The former primary magmatic mineral parageneses for the meta-mafic enclaves and the meta-plagiogranites are similar and consist of An-rich-plagioclase + hornblende + ilmenite + monazite + apatite. However, the Ab-rich plagioclase abundance of the meta-mafic enclave is much lower and the hornblende and accessory mineral abundances are higher compared with those of the meta-plagiogranite host rocks. Meta-plagiogranites with mafic enclaves have a chemical composition similar to that of typical adakites, with elevated Al 2O 3 (15.86 wt.%), Mg# (53.5) and Sr contents (485 ppm) and a positive Eu anomaly. The rare earth element patterns are highly fractionated with ((La/Yb) N> 12) and low Y (< 4 ppm) and Yb (< 0.4 ppm). The mafic enclave has a basaltic composition with a negative Eu anomaly and has ca. 10 times higher HREE and Y concentrations than the plagiogranite host rocks. Enclave-free meta-plagiogranites also have high Al 2O 3 (16.08 wt.%), Na 2O (8.88 wt.%) and Mg# (52.3) values without obvious Eu enrichment or depletion. The Y and Yb concentrations (13 and 1.33 ppm, respectively) are higher than those of the enclave-bearing meta-plagiogranites and slightly lower than those of the mafic enclave. Zircon of the enclave-bearing meta-plagiogranites gives a weighted mean 206Pb/ 238U crystallization age of 970 ± 21 Ma. These meta-plagiogranites have low 87Sr/ 86Sr value of ~ 0.7026 and high εNd(970 Ma) of 6.6 to 7.4. The meta-mafic enclave has a similar isotope composition with a 87Sr/ 86Sr value of 0.7023 and a εNd( T) of 6.2. The enclave-free meta-plagiogranites have relatively higher 87Sr/ 86Sr (0.7037 to 0.7045) and lower εNd( T) (− 1.22 to − 1.28) values. Trace element data of the minerals and whole rocks suggest that Y and Yb are hosted in amphibole, apatite and titanite, Nb in rutile, titanite and amphibole and Sr mainly in plagioclase. Petrographic and geochemical evidence indicates that the meta-mafic enclaves are ‘schlieren’, which formed by the early crystallization of accessory minerals and hornblende, in the meta-plagiogranites. The adakitic signature of enclave-bearing meta-plagiogranites is interpreted to be due to fractional crystallization of hornblende and accessory minerals such as apatite and ilmenite from the parent magmas, instead of the melting of subducted, spilitized oceanic crust with residual garnet and amphibole. The plagiogranitic magma may have evolved from a basaltic parent magma which had originated in the mantle wedge beneath the Neoproterozoic ( ca. 970 Ma) island arc along the southern margin of the Yangtze block.