Geochemical insights into the role of metasomatic hornblendite in generating alkali basalts

Abstract

Experimental petrology suggested the role of hornblendite in generating alkali basalt. This mechanism is confirmed by an integrated study of major-trace elements and radiogenic isotopes for Mesozoic alkali basalts from the Qinling orogen in China. The alkali basalts have high contents of MgO (4.8-11.1 wt %, Mg# 5 47-69), Na 2 O 1 K 2 O (2.9-5.4 wt %), TiO 2 (2.0-3.1 wt %) but low content of SiO 2 (41.4-49.6 wt %), which are generally silica-undersaturated with normative minerals of nepheline and olivine. They exhibit OIB-like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE relative to the primitive mantle. They also show relatively depleted Sr-Nd-Hf isotope compositions, with low initial 87 Sr/ 86 Sr ratios of 0.7028-0.7058, positive e Nd (t) values of 4.0-9.8 and e Hf (t) values of 8.8-13.5 for whole-rock, and positive e Hf (t) values of 5.2-16.4 for zircon. Such element and isotope features indicate their origination from the juvenile subcontinental lithospheric mantle (SCLM) source with involvement of crustal components. The alkali basalts generally have high K 2 O/Na 2 O ratios, and high K 2 O and TiO 2 contents, suggesting their derivation from partial melting of hornblendite-rich mantle lithology. They also exhibit variable K/La and Ti/La ratios that are correlated with (La/Yb) N ratios, indicating a geochemical heterogeneity of the SCLM source. Taken together, all the above geochemical features can be accounted for by partial melting of a hornblendite-rich SCLM source. The hornblendite would be generated by reaction of the juvenile SCLM wedge peridotite with hydrous felsic melts derived from subducted Palaeotethyan oceanic crust at the slab-mantle interface in the subduction channel. Therefore, orogenic alkali basalts record recycling of the subducted fossil oceanic crust, and the metasomatic hornblendite is an important lithology in local SCLM domains above fossil subduction channels.

Early Paleozoic granitic magmatism related to the processes from subduction to collision in South Altyn, NW China

“…Generally, continental crust often begins to subduct/collide after the oceanic subduction due to the pulling force during the whole orogenic process (Ye et al, 2000;Dai et al, 2014Dai et al, , 2015Li, 2014;Turcotte and Schubert, 2014;Zhou et al, 2015). Being a typical continental subduction/collision orogen, HP-UHP rocks from South Altyn were shown by previous studies to have experienced continental deep subduction and two subsequent uplifts .…”

Magmatism, mantle evolution and geodynamics at the converging plate margins of Italy

“…Sobolev et al 2005;Herzberg 2011), and melting of amphibole-rich lithologies in the lithosphere or at the lithosphere-asthenosphere boundary (e.g. Pilet et al 2011;Dai et al 2014).…”

Two types of the crust-mantle interaction in continental subduction zones

“…Reaction of these melts with the overlying mantle wedge peridotite would generate pyroxene/amphibole-rich metasomatites. The metasomatites could be stored in the lithospheric mantle for a long time, and then become partially melted to give rise to mafic igneous rocks, which have OIB-like trace element distribution patterns, depleted radiogenic isotope compositions and H-O isotope compositions different from those of the normal mantle (Zheng, 2012;Dai et al, 2014Dai et al, , 2015aDai et al, , 2015bXu et al, 2014aXu et al, , 2014b.…”

“…The produced felsic melts are remarkably enriched in LILE and LREE but depleted in HFSE and HREE due to the stability of rutile and garnet during the dehydration melting. If the accessory minerals of rutile and garnet decompose, the resulted melts would be not depleted and even enriched in HFSE and HREE (Ringwood, 1990;Zheng, 2012;Dai et al, 2014Dai et al, , 2015b. In any case, these felsic melts would metasomatize the overlying mantle wedge above subduction channel, generating the mantle sources of mafic igneous rocks in deep subduction zones.…”

“…Melts produced by partial melting of subducted oceanic crust at mantle depths of >120 km are generally characterized by OIB-like trace element distribution patterns (due to breakdown of rutile) and depleted Sr-Nd-Hf isotope compositions (Ringwood, 1990;Zheng, 2012;Dai et al, 2014), and the overlying mantle wedge is juvenile SCLM formed by cooling of the asthenospheric mantle ( Figure 6). In contrast, melts generated by partial melting of subducted continental crust generally exhibit arc-like trace element distribution patterns and enriched Sr-Nd-Hf isotope compositions , and the overlying mantle wedge is ancient cratonic SCLM ( Figure 7).…”