Origin of Granitic Magma within continental margin arc magmatic lull: constraints by the Late Cretaceous granites in Tengchong Block, SE Tibet

    Magmatism in continental margin arc is generally episodic, and alternatively appearance with magmatic flare-up and lull. Accompanying the magmatic flare-up and lull, the arc could move toward the oceanic trench or continental. Although numerous researches are related to arc tempos, there is little understanding for the genetic mechanism of magmatism during lull. According to the distribution characteristics of magmatic rocks in the Neo-Tethyan continental margin arc, this study selected the Late Cretaceous granitic rocks in Tengchong block to further constrain their petrogenesis and dynamics process during lulls. The monzogranite ages in Husa area of Tengchong Block are 73Ma, formed within the Late Cretaceous magmatic lull (85-65Ma) at the continental margin arc related to Neo-Tethyan subduction. They are metaluminous to weakly peraluminous and calc-alkaline, with enriched Nd-Hf isotopic characteristics. These signatures reveal their sources are orthometamorphic gneisses in the Gaoligongshan Formation. In addition, the continental margin arc related to the flat subduction of Neo-Tethyan slab should be compressional state during Late Cretaceous (ca. 85-65Ma). Comparing Late Cretaceous magmatic rock assemblages, geochemical signatures, Zr saturated temperature and zoning in plagioclase with these in Early Eocene rocks during magmatic flare-up (ca. 55-50Ma), the granitic rocks during magmatic lull formed at a thicken crust and partial melting of gneisses induced by a long-time thermal accumulation. Therefore, the continental margin arc could form granitic magma by a long-time thermal accumulation within a thicken crust during magmatic lull with the low mantle-derived magma flux at a local region.

Supported by National Natural Science Foundation of China [Grant Nos. 42272052 and 41902046].