Arc Andesitic Rocks Derived From Partial Melts of Mélange Diapir in Subduction Zones: Evidence From Whole‐Rock Geochemistry and Sr‐Nd‐Mo Isotopes of the Paleogene Linzizong Volcanic Succession in Southern Tibet

Abstract

Formation of andesitic rocks is crucial to understand the evolution of continental crust.Here we present an integrated study for the Paleocene Dianzhong Formation of the Linzizong volcanic succession (LVS) in the Lhasa terrane, southern Tibet. The andesites of the Dianzhong Formation have relatively high Mg # (38-60) and exhibit typical subduction-like geochemical signatures. They are characterized by slightly enriched ε Nd (t) values (−2.82-0.35) and have a range of δ 98 / 95 Mo isotopic compositions (−0.40 to 0.13 ‰) larger than mid-ocean ridge basalt. Their high Mg # , moderate light rare earth element-enriched patterns, and slightly enriched ε Nd (t) values clearly rule out an origin of continental crust-derived melts or partial melts of earlier underplated mafic rocks. The dacites show similar geochemistry to the andesites, indicating an origin from the parental magmas of the andesites. Based on the geochemical comparisons between the Dianzhong andesites and rocks of mélange, we suggest that the andesitic rocks of the LVS were derived from partial melts of a mantle source dominated by mélange that formed along the subduction channel. The mélange diapir melting was triggered by asthenospheric upwelling and corner flow during the Paleocene slab roll-back of northward subducted Neotethyan oceanic crust. Because the Early Paleocene andesitic rocks in southern Lhasa show an average composition of continental crust, the partial melting of mélange rocks most likely provide an important mechanism for the production of andesitic magmas within continental arcs.

Microcontinent subduction and S-type volcanism prior to India–Asia collision

“…Previous studies consider the Dianzhong volcanic rocks as I-type rocks originated from mantle wedge that containing some crustal component followed by the AFC processes [5][6][7]9 . However, these S-type volcanic and magmatic rocks have extremely low Na 2 O, CaO , MgO, FeO T , Cr (5.20-15.42), Ni (1.09-3.57), and Co (0.60-1.54) contents with radiogenic Sr and Pb isotope composition compared with Dianzhong I-type series 5,6,8,9 , which indicate that they are not likely originated from partial melting of the mantle wedge or the mafic lower continental crust (LCC). Besides, the initial 87 Sr/ 86 Sr ratios and ε Nd (t) of the granitoids and volcanic rocks exhibit inconsistent relations rather than any correlation with SiO 2 contents (Fig.…”

“…As evident from their thicknesses, the Dianzhong and Pana Formations indicate more intense volcanic activity than the Nianbo Formation. The Dianzhong and Nianbo Formations show arc-like geochemical signature with significant mantle contributions [5][6][7][8][9]13 , in contrast to the geochemically heterogeneous of the Pana Formation 7,9 . The same inference can also be made for the coeval intrusive rocks as they show similar whole-rock geochemistry and Sr-Nd isotope, and zircon Hf isotope compositions with the Cretaceous I-type Gangdese batholith 6,[14][15][16] .…”

“…The Linzizong volcanic succession (LVS) and the coeval intrusive rocks in southern Tibet, which cover more than 50% of the Gangdese Belt extending E-W for more than 1200 km (Fig. 2A) are prominent markers of the magmatic activity associated with the India-Asia collision [4][5][6][7][8][9] .…”

“…The wt%) contents compared with the normal Dianzhong magmatic rocks 5,6,8,9 . The volcanic rocks and granitoids plot within the rhyolite and granite fields, respectively, on the total alkalis vs. silica (TAS) diagram (Fig.…”

Numerical modeling of subduction: State of the art and future directions

“…Stable arcs showed the emplacement of flattened intrusions in the lower crust characterized by a successively increasing mantle component and low magmatic addition rates (10 km 3 /km/m.y.). Compressional arcs showed frequent rising of partially molten diapirs (e.g., Tamura, 1994;Hall and Kincaid, 2001;Gerya and Yuen, 2003;Zhu et al, 2009;Behn et al, 2011;Marschall and Schumacher, 2012;Yan et al, 2019;Zhang et al, 2020;Lin et al, 2021) and higher magmatic addition rates (40-70 km 3 /km/m.y.) compared to stable arcs.…”