Extremely high-Na adakite-like magmas derived from alkali-rich basaltic underplate: The Late Cretaceous Zhantang andesites in the Huichang Basin, SE China.

Abstract

The Zhantang andesites, which erupted in the Late Cretaceous Huichang Basin of SE China interior, were associated with the Late Mesozoic lithospheric extension and basaltic underplating. They are highNa (6.59-8.46 wt% Na 2 O), high-Al trondhjemitic rocks with high Sr and Ba, low Y and HREE, and high Sr/Y and La/Yb ratios, similar to modern adakites. However, they are much higher in Na 2 O but lower in CaO than adakites. Their εNd(T) values of -2.3 to -3.8 and initial 87 Sr/ 86 Sr ratios of 0.707 to 0.708 are also significantly different from those of adakites with MORB-like isotopic compositions. These chemical and isotopic features, along with the tectonic setting, suggest that they were not produced by partial melting of subducting slab.The Zhantang adakite-like magmas may be derived from underplated basaltic lower crust, but require a compositionally peculiar basaltic protolith to account for their extremely high-Na and low-Ca feature. They are compositionally comparable to the experimentally produced partial melts of alkali-rich basalt. We therefore interpret them as the products of melting of an alkali-rich basaltic rock.

Sr–Nd isotope geochemistry and tectonomagmatic setting of the Dehsalm Cu–Mo porphyry mineralizing intrusives from Lut Block, eastern Iran

“…These characteristics could be a consequence of melting of garnet amphibolite or eclogite facies rocks that may be found in subducted oceanic crust (Defant and Drummond 1990). However, other sources of adakitic parental magmas have been proposed, such as hydrous mantle peridotite (Stern and Hanson 1991), mafic rocks at the base of thickened lower crust (Zhang et al 2001;Chung et al 2003;Xiong et al 2003;Hou et al 2004;Wang et al 2005;Guo et al 2006) or delaminated mafic lower crust (e.g., Defant et al 2002;Gao et al 2004;Guo et al 2006;Lai et al 2007;Liu et al 2008a, b). Some authors (e.g., Castillo et al 1999;Macpherson et al 2006) consider that assimilation-fractional crystallization (AFC) processes must be taken into account to explain the genesis of the adakitic rocks.…”

Mesozoic adakites in the Lingqiu Basin of the central North China Craton: Partial melting of underplated basaltic lower crust

“…3A). Their very low Rb/Sr ratios (0.03 to 0.07) suggest little, if any, fractional crystallization of plagioclase (Xiong et al, 2003). In the Sr/Y-Y diagram, the Baiqi volcanic rocks fall into the field of typical adakites and TTG, contrasting the arc andesite and dacites (Fig.…”

“…Thus, we term the Baiqi volcanics as adakitc rocks in the following discus- (Rapp, 1995;Rapp et al, 1991Rapp et al, , 1999Rapp and Watson, 1995); R 2-4 SW = Partial melts of basalts compositionally close to N-MORB (Rapp and Watson, 1995;Rapp et al, 1991;Sen and Dunn, 1994;Winther, 1996); WW = Partial melts of low-K and Na, high-Mg and Ca basalt (Wolf and Wyllie, 1994). The arc-related adakites (grey field) following Xiong et al (2003).…”

“…Partial melting of basaltic rocks can produce adakitic melt at pressures equivalent to a crustal thickness of >40 km (Rapp and Watson, 1995;Rapp et al, 1991). This rock attracts widespread attention due to its significance in revealing deep geodynamic processes, e.g., oceanic crust subduction (Defant and Drummond, 1990;Martin et al, 2005;Martin, 1999;Peacock et al, 1994;Stern and Kilian, 1996), basalt underplating (Atherton and Petford, 1993;Petford and Atherton, 1996;Rapp and Watson, 1995;Xiong et al, 2003), recycling of lower continental crust Kay and Kay, 1991;Xu et al, 2002), melt-peridotite reaction (Liu et al, 2005; rocks featured by high Mg# were interpreted as foundering lower continental crust-derived melt interacted with peridotites in the mantle Xiao et al, 2004). Despite these investigations, some questions remain: how were these adakitic rocks related to the foundering lower continental crust or basalt underplating?…”

Late Early Cretaceous adakitic granitoids and associated magnesian and potassium‐rich mafic enclaves and dikes in the Tunchang–Fengmu area, Hainan Province (South China): Partial melting of lower crust and mantle, and magma hybridization

“…In recent years, many Jurassic-Cretaceous adakitic intrusive rocks (intermediate-acidic rocks characterized by a depletion of Y and heavy rare earth elements (HREEs), and high Sr/Y and La/Yb ratios) have been documented in the eastern Yangtze Block (e.g., Xu et al, 2002a;Wang et al, 2004aWang et al, , b, 2006aHuang et al, 2008;Xie et al, 2008;Li et al, 2009;Ling et al, 2009;Liu et al, 2010a;Sun et al, 2011). Only very minor occurrences of adakitic rocks have been reported in the central part of the Cathaysia Block, (e.g., Xiong et al, 2003), however, despite the widespread occurrence of Mesozoic intrusive rocks in the region. In addition, few mafic enclaves have been reported from adakitic plutons in the eastern Yangtze and central Cathaysia Block.…”