Calcium isotope composition of the Gangdese continental arc deep crust and implications for making continental crust

Magmatism in deep continental arcs is crucial for understanding the formation of andesitic continental crust. Garnet, pyroxene and amphibole are typical major mafic phases in deep arcs, with garnet potentially causing noticeable variations in stable isotopes of major elements such as Ca (δ^44/40Ca). The upper continental crust (UCC) shows low δ^44/40Ca value (0.70 ± 0.02 ‰, 2SE) relative to oceanic crust (0.85 ± 0.09 ‰), which is inferred to result from accumulation of garnet-rich mafic rocks with high δ^44/40Ca. However, due to limited exposure of middle-lower arc crustal sections, the δ^44/40Ca values of the deep continental crustal rocks are lacking, and the effect of magmatism in the deep crust is poorly understood. A juvenile (90–80 Ma) and continuous (∼ 42 to 17 km in depth) continental arc lower crust section is exposed in the Gangdese arc, southern Tibet. This section comprises garnet metagabbros, overlying mafic-intermediate sequence with a minor later ultramafic sequence. Here we present the δ^44/40Ca of these rocks and mineral separates to understand the Ca isotope compositions of deep continental arc crust.

The garnet metagabbros were formed from granulite-facies metamorphism of cumulative gabbros, which overall exhibit low δ^44/40Ca (from 0.59 ‰ to 0.85 ‰, with a mean value of 0.71 ± 0.06 ‰, 2SD, N = 10). Two samples with higher δ^44/40Ca values could be ascribed to higher proportions of coarse garnet grains in analyzed fractions. The mafic-intermediate rocks with different chemical compositions show similar δ^44/40Ca (0.58–0.72 ‰, with an average of 0.66 ± 0.06 ‰, 2SD, N = 12). The ultramafic sequence with variable degree of accumulation also displays consistent δ^44/40Ca (0.68–0.82 ‰). These results indicate limited Ca isotope fractionation (< 0.1 ‰) during the continental arc magmatism related to pyroxene, plagioclase and amphibole. Based on these lithologies, the Gangdese continental deep crust of variable depths shows a mean δ^44/40Ca value of 0.69 ± 0.07 ‰, indistinguishable from the upper crust. The estimated δ^44/40Ca for the continental crust is thus lighter by ∼ 0.1 ‰ than the oceanic crust and by ∼ 0.2 ‰ than the mantle. Such difference could be explained by crystallization and segregation of garnet-bearing (∼ 20 %) cumulates in deeper Gangdese arc crust, consistent with high (Dy/Yb)_N of the most primitive mafic-intermediate rocks relative to MORBs (e.g., 1.4 versus 1.1). Therefore, our data supports the importance of accumulation of garnet-rich mafic rocks in the formation of thick continental arc crust. Conversely, island arc lavas show MORB-like δ^44/40Ca, indicating the negligible effect of garnet during island arc magmatism. The distinct δ^44/40Ca of the continental crust from the oceanic crust and island arc magmas suggests the critical role of garnet in the formation of continental crust.