Andesitic arc magmas derived from two contrasting melange origins: Evidence from central Tibetan dioritic porphyries

CHEMICAL GEOLOGY(2024)

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摘要
Numerous studies have argued that the andesitic model (i.e., the arc directly produces andesitic magmas) rather than the basaltic-input model (i.e., andesitic magmas were derived by fractionation from basaltic magmas) better explains the composition of continental crust. However, it remains controversial as to how andesitic magmas are directly produced in subduction zones, and this weakens the basis of the andesitic model. Recently, the m & eacute;lange-diapir model has been increasingly considered to be an important mechanism for the direct generation of arc andesites, and would thus support the andesitic model for crustal growth. Moreover, melting experiments on m & eacute;lange rocks show that varying the composition of m & eacute;lange rocks (e.g., serpentinite-dominated and sediment-dominated m & eacute;lange) can produce a range of primary andesitic arc magmas, from tholeiitic, calc-alkaline, to high-K calc-alkaline and shoshonitic magma types. However, it remains challenging to determine the different m & eacute;lange sources for low-K to high-K arc andesites. Here we verify these experimental results using data from early Cretaceous dioritic porphyries in the southern Qiangtang block of central Tibetan plateau. The generation of these dioritic rocks cannot be linked to any coeval basaltic rocks in this region and so they may represent primary andesitic magmas. These dioritic porphyries are divided into low-K type-1 and high-K type-2. Type-1 has higher delta Mg-26 values (-0.16 to -0.08 parts per thousand) than MORBs (mid-ocean ridge basalt, delta Mg-26 = -0.25 +/- 0.06 parts per thousand), which can be ascribed to the contribution of subducted bulk/un-differentiated serpentinite in their source. Combined with their low K contents and La/Sm ratios, and high NdHf isotope ratios (epsilon Nd(t) = 3.36-5.03; epsilon Hf(t) = 12.88-13.76), we suggest a serpentine-dominated m & eacute;lange for their origin. The type-2 and type-1 rocks have similar trace-element distribution patterns, the same ages (similar to 124 Ma), and are indistinguishable in field outcrops, indicating their common petrogenesis. However, compared to type-1, type-2 has higher K and Th contents, and La/Sm and Th/Nd ratios with lower NdHf isotope values (epsilon Nd(t) = -1.61 to +0.30; epsilon Hf(t) = 3.78-4.39), and mantle-like delta Mg-26 values (-0.28 to -0.17 parts per thousand). This likely indicates derivation from a sediment-dominated m & eacute;lange for type-2. This study thus verifies the experimental results of melting of different m & eacute;lange from natural rock record and shows that the m & eacute;lange model provides an important mechanism for generating variable andesitic arc magmas and so the formation of the andesitic continental crust.
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关键词
Subductionmelange,Andesitic arc magmas,Crustal growth,Mg isotopes,Tibetan plateau
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