Coeval Mantle-Derived and Crust-Derived Magmas Forming Two Neighbouring Plutons in the Songpan Ganze Accretionary Orogenic Wedge (SW China)

The Songpan Ganze accretionary orogenic wedge (Eastern Tibetan Plateau, China) is characterized by midto late Triassic syn-orogenic magmatism displaying a diversity of petrological and geochemical characteristics: I-type lowto high-K calc-alkaline, A-type alkaline, and S-type peraluminous granitoids. To better understand the genesis and emplacement of magmas associated with tectonic accretion along convergent plate boundaries, we present a comprehensive petrological, geochemical, and geochronological study of the neighbouring Markam and Taiyanghe granitoid plutons (separated by less than 2 km) intrusive into Songpan Ganze metasediments. The main volume of the Markam pluton is dominated by an S-type peraluminous mediumto coarse-grained granite [QtzþPlþKfs(6Mc)þBtþMsþGrtþChlþTur] which was emplaced over a period of 20 Myr from 231 to 209 Ma based on U–Pb zircon ages, at an estimated minimum depth of 11 km (300 MPa). Markam granites [0 709405< Sr/Sr(t)< 0 711113; –9 02< eNd(t)< –7 99] result from the melting of the Songpan Ganze metasediments under amphibolite-facies conditions at a depth of at least 20 km. The magmatic series has evolved from the parental melt by fractional crystallization yielding a residual S-type, peraluminous, coarse-grained granite with feldspar phenocrysts (QtzþKfsþPlþBtþMsþAln) forming the southeastern branch of the Markam pluton with U–Pb zircon ages ranging from 230 to 200 Ma (peak at 207 Ma). The main volume of the Taiyanghe pluton is dominated by I-type, high-K calc-alkaline, fineto coarse-grained diorite and syenodiorite (QtzþPlþKfsþBtþAmp 6 Chl 6 Grt), and was emplaced over a period of 40 Myr (239–202 Ma) at an estimated minimum depth of about 11 km. Taiyanghe rocks have isotopic signatures [0 708037< Sr/Sr(t)< 0 708665; –6 27< eNd(t)< –3 62] typical of magmas resulting from partial melting in a subduction context of an ultramafic source under eclogite-facies conditions and were only slightly geochemically influenced by Songpan Ganze metasediments. This magmatic series has experienced limited fractional crystallization from the parental magma, leading to the formation of a (PlþAmp)-rich cumulate. The Taiyanghe pluton also includes I-type, low-K calc-alkaline hornblende-gabbros [Amp(Hbl)þPlþQtzþBt], that have to our knowledge not been recognized before in the Songpan Ganze terrane, which we interpret as a residual mush resulting from the differentiation of the Taiyanghe magmas, intruding the pluton soon after its formation. These data provide evidence that the Triassic plutons intruding the Songpan Ganze accretionary orogenic wedge result from partial melting at different levels of mantle and crustal protoliths leading to magmas that show limited interactions despite their nearby emplacement, over a 30 Myr period. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup. comã 2221 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 11, 2221–2256 doi: 10.1093/petrology/egy007 Advance Access Publication Date: 3 February 2018