Cenozoic K-rich adakitic volcanic rocks in the Hohxil area, northern Tibet: Lower-crustal melting in an intracontinental setting

Abstract

It is generally accepted that the Cenozoic potassic volcanic rocks of northern Tibet were derived from a lithospheric mantle source. Here we report new chronological, geochemical, and isotopic data for the Miocene (ca. 18-15 Ma) K-rich adakitic volcanic rocks from the Hohxil area of the Songpan-Ganzi block in northern Tibet. We contend that these rocks were generated by partial melting of the mafic lower crust, in an intracontinental setting unrelated to subduction of oceanic crust. The Hohxil rocks exhibit high Sr/Y and La/Yb ratios, high Sr and La contents, but low Yb and Y concentrations, similar to adakites formed by slab melting associated with subduction. However, their relatively low Nd values (؊2.09 to ؊4.58); high 87 Sr/ 86 Sr (0.7072-0.7075), Th/U, Th/Ba, and Rb/Ba ratios; and distinctive potassium enrichments (K 2 O Ͼ Na 2 O) are very different from the composition of typical adakites. In addition, those K-rich adakitic rocks with the highest SiO 2 contents (Ͼ61 wt%) exhibit the lowest 87 Sr/ 86 Sr ratios and highest Nd values and are the oldest Cenozoic volcanic rocks exposed in the Songpan-Ganzi block, suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma. Taking into account the composition of lower-crustal mafic xenoliths in Tibet, as well as the tectonic and geophysical evidence, we conclude that the Hohxil adakitic magmas were produced by partial melting of amphibole-bearing eclogites with a K-rich mafic bulk composition, in the lower part (Նϳ55 km) of the thickened northern Tibetan crust. Partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward-subducted continental lithosphere.

Overlapping Sr–Nd–Hf–O isotopic compositions in Permian mafic enclaves and host granitoids in Alxa Block, NW China: Evidence for crust–mantle interaction and implications for the generation of silicic igneous provinces

“…Studies reporting the generation of related felsic magmas (Atherton and Petford, 1993;Petford and Atherton, 1996) are comparatively rare, although basaltic magma underplating (Annen et al, 2006;Li et al, 2014;Petford and Gallagher, 2001) and partial melting of normal (Bryan et al, 2002) or thickened mafic lower crust (Chung et al, 2003;Johnson et al, 1997;Wang et al, 2005Wang et al, , 2006a have been documented in many cases. The difficulties in identifying magmas sourced from such newly-formed mafic rocks likely occur because the volumes of magma are relatively small in volume and may be masked by the predominance of ancient crust-derived magmas or mantle-derived magmas, and/or due to the lack of isotopic contrast between the mantle and the new crustal sources (Frost et al, 2001).…”

An Early Permian (ca. 280Ma) silicic igneous province in the Alxa Block, NW China: A magmatic flare-up triggered by a mantle-plume?

“…Partial melting of a thickened garnet-bearing mafic lower crust due to heat flux from the mantle (e.g., Atherton and Petford, 1993;Wang et al, 2005Wang et al, , 2007b) is a plausible mechanism for the generation of the high Sr/Y granodiorites. If the high Sr/Y magmas are actually derived from the mafic lower crust, then they should have relatively low MgO contents and be compositionally similar to 1-4.0 GPa experimental melts of metabasalts.…”

“…Fields of adakites inferred to be derived from subducting oceanic crust is after Wang et al (2006). Melts formed by partial melting of the lower mafic crust are from the following references: Atherton and Petford (1993);Chung et al (2003); Johnson et al (1997); Muir et al (1995); Petford and Atherton (1996);Wang et al (2005). (f) Nd-Sr isotope composition for the Early Permian igneous rocks.…”

“…Thus, they should be classed as high Sr/Y granites (Moyen, 2009). Several models have been proposed for the generation of high Sr/Y granitoids, including (a) melting of subducted young and hot oceanic crust (Defant and Drummond, 1990;Kay et al, 1993;Martin et al, 2005;Stern and Kilian, 1996;Tang et al, 2010a;Wang et al, 2007aWang et al, , 2008; (b) assimilation and fractional crystallization (AFC) or fractional crystallization (FC) from parental basaltic magmas (Castillo et al, 1999;Macpherson et al, 2006;Richards and Kerrich, 2007;Rooney et al, 2011); (c) magma mixing between felsic and basaltic magmas (Guo et al, 2007;Streck et al, 2007); and (d) partial melting of thickened lower crust (Atherton and Petford, 1993;Chung et al, 2003;Petford and Atherton, 1996;Wang et al, 2005Wang et al, , 2007b.…”

Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: Partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet

“…51 Ma) adakitic rocks in the Quxu and Nyingchi areas Zhang et al, 2010a). Adakitic rocks may be generated by a variety of mechanisms (Castillo, 2012), such as melting of subducted young and hot oceanic crust (Defant and Drummond, 1990), partial melting of thickened basaltic lower crust (Atherton and Petford, 1993;Chung et al, 2003;Wang et al, 2005), partial melting of subducted continental crust , crustal assimilation and low-pressure fractional crystallization from parental basaltic magmas (Castillo et al, 1999), high-pressure crystallization (involving garnet) of mafic magmas derived from mantle wedge peridotites (Macpherson et al, 2006), magma mixing between felsic and basaltic magmas (Streck et al, 2007), and partial melting of tonalitic to granodioritic crust (Kamei et al, 2009). We evaluate these alternative processes in the following sections with specific reference to the Napuri adakitic rocks.…”