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

• Published in: Lithos 2009-06, Vol.110 (1), p.277-293
• Main Authors: Gao, Jun, Klemd, Reiner, Long, Lingli, Xiong, Xianming, Qian, Qing
• Format: Article
• Language: English
• Subjects: Adakitic signature; Fractional crystallization; Neo-Proterozoic meta-plagiogranites; The NE Jiangxi ophiolitic mélange belt
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2009.01.009

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 adakiti