Geochemistry and spatial distribution of OIB and MORB in A’nyemaqen ophiolite zone: Evidence of Majixueshan ancient ridge-centered hotspot

Abstract

The mafic volcanic association is made up of OIB, E-MORB and N-MORB in the A'nyemaqen Paleozoicophiolites. Compared with the same type rocks in the world, the mafic rocks generally display lower Nb/U and Ce/Pb ratios and some have Nb depletion and Pb enrichment. The OIB are LREE-enriched with (La/Yb) N =5-20, N-MORB are LREE-depleted with (La/Yb) N = 0.41-0.5. The OIB are featured by incompatible element enrichment and the N-MORB are obviously depleted with some metasomatic effect, and E-MORB are geochemically intermediated. These rocks are distributed around the Majixueshan OIB and gabbros in a thickness greater than a thousand meters and transitionally change along the ophiolite extension in a west-east direction, showing a symmetric distribution pattern as centered by the Majixueshan OIB, that is, from N-MORB, OIB and E-MORB association in the Dur'ngoi area to OIB in the Majixueshan area and then to N-MORB, OIB and E-MORB assemblage again in the Buqingshan area. By consideration of the rock association, the rock spatial distribution and the thickness of the mafic rocks in the Majixueshan, coupled with the metasomatic relationship between the OIB and MORB sources, it can be argued that the Majixueshan probably corresponds to an ancient hotspot or an ocean island formed by mantle plume on the A'nyemaqeh ocean ridge, that is the ridge-centered hotspot, tectonically similar to the present-day Iceland hotspot.A'nyemaqen ophiolite zone, OIB, N-MORB and E-MORB association, spatial distribution, Majixueshan ridge-centered hotspot, metasomatism, mantle plume Coexistence of normal mid-ocean-ridge basalts (N-MORB), enriched mid-ocean-ridge basalts (E-MORB) and ocean island basalts (OIB) in the certain tectonic setting has drawn extensive attention and strong interest. Iceland in the North Atlantic Ocean has become a well-known example for the phenomenon. The phenomenon has been thought to be a result of superposition of a hotspot on a ridge (ridge-centered hotspot) [1] or an oceanic island on a ridge (ridge-centered island) [2] or a mantle plume on a ridge (plume on-ridge) [3] , and hotspot-ridge interaction or plume-ridge interaction [4,5] . The spatial superposition and materials interaction between a ridge and a hotspot or an ocean island or a plume cause anomalies of geochemistry, topography, crustal structure, gravity, seismic velocity and bathymetry in the area surrounding a ridge-hotspot [1] .Recent studies have indicated that the coexistence not only occurs in a modern environment like Iceland, but also can be traced into ophiolite zones representing preserved oceanic crust in continental orogenic belts. Hou et al. [6,7] , according to the coexistence of OIB, N-MORB and T-MORB and their specific configuration in space, proposed a paleo-Tethyan mantle plume model to inter-

Recognizing OIB and MORB in accretionary complexes: A new approach based on ocean plate stratigraphy, petrology and geochemistry

“…Triangle tectonic discrimination diagrams, MnO-TiO 2 -P 2 O 5 (Mullen, 1983); B, Zr/4-2Nb-Y (Meschede, 1986); C, Zr-Ti/100-3Y (Pearce and Cann, 1973), for basalts from accretionary complexes of Japan (upper row) and for the Archean, Paleozoic and Meso-Cenozoic MORBs (middle row) and OIBs (lower row). The lower two rows show differences between Archean and Phanerozoic MORBs and OIBs and thus imply uncertainties in their application (based on data from Dong et al, 2008;Gordienko et al, 2012;Guo et al, 2007;Ichiyama et al, 2014;Jian et al, 2010;Koizumi and Ishiwatari, 2006;Komiya et al, 2002Komiya et al, , 2004Kuzmichev et al, 2005;Mahoney et al, 2005;Miao et al, 2008;Miyake, 1985;Mizoguchi et al, 2009;Mongush et al, 2011;Regelous et al, 1999;Safonova et al, 2008Safonova et al, , 2009Safonova et al, , 2011aSafonova et al, ,b, 2012Safonova et al, , 2015aTatsumi et al, 1998Tatsumi et al, , 2000Tejada et al, 1996;Ueda and Miyashita, 2005;Volkova and Budanov, 1999;Zhang et al,2009 shown in Suppl. Electr.…”

Geochemistry and spatial distribution of late-Paleozoic mafic volcanic rocks in the surrounding areas of the Gonghe Basin: Implications for Majixueshan triple-junction and east Paleotethyan archipelagic ocean

“…Also, about 1000-m-thick basaltic lava and stratiform gabbros are exposed in the Qianliwalima area to southwest of the Majixueshan area. Guo et al identified OIB in association with N-MORB and T-MORB in both Dur'ngoi and Buqingshan areas and suggested that the A'nyêmaqên ophiolite zone represents an Iceland-like ancient ridge-centered hotspot [8] .…”

“…The spatial distribution of the mafic rocks is an analogue of today's North Atlantic Iceland ridge-centered hotspot [8] .…”

“…The triple-junction first rifted on the Precambrian and Caledonian continental basement, which was marked by the Devonian diabase dykes [12] . And then, during the late Devonian and early Carboniferous, around the Majixueshan area, the oceanic crust occurred in the eastern and western arms of the junction, also in the southern potion of the KS zone [6][7][8][9]14] that is a failed northern arm of the junction and the main section is the continental rift setting. In terms of tectonics, the formation of the triple-junction is an expression of hotspot activities that initiated the continental breakup and continent-ocean transformation.…”

“…Rehkamper et al suggested that low Nb/U and Ce/Pb ratios and high 87 Sr/ 86 Sr ratios in the Central Indian and Carlsburg Ridge in the Indian Ocean characterize the enriched end member that results in crustal contamination in the upper mantle MORB source region [23] . The low Nb/U and Ce/Pb ratios in the A'nyêmaqên MORB are thought to relate to the metasomatism and contamination of the Majixueshan OIB [8] . There is no other late-Paleozoic OIB occurrence in the adjacent area of the ZWL zone, thus, like the A'nyêmaqên MORB, the ZWL MORB may have experienced the same metasomatism by the Majixueshan OIB materials.…”