Sources of Quaternary Potassic Volcanic Rocks From Wudalianchi, China: Control by Transtension at the Lithosphere–asthenosphere Boundary Layer

Abstract

Introduction. Transtension is a system of stresses that tends to cause oblique extension, i.e. combined extension and strike slip. Syn-volcanic transtensional deformations of the lithosphere may provide two possible scenarios for control of magmatic processes. One scenario assumes ascending sub-lithospheric melts that mark the permeable lithosphere in a transtension area without melting of the lithospheric material; products of volcanic eruptions in such a zone show only the sub-lithospheric mantle material; components of magmatic liquids do not reveal any connection to the lithospheric structure. Another scenario yields a direct control of melting in lithospheric sources in an evolving transtensional structure. In this case, spatial-temporal changes of lithospheric and sub-lithospheric components are a direct indication of the evolving transtensional zone. In this paper, we present arguments in favor of the transtensional origin of the lithosphere-derived melting anomaly along the Wudalianchi volcanic zone, which are based on the study of components in the rocks sampled from the volcanic field of the same name. Analytical methods. Trace elements were determined by ICP-MS using a mass-spectrometer Agilent 7500ce and isotopes using a mass-spectrometer Finnigan MAT 262. The methods used were described in the previous papers by Rasskazov et al. [2011] and Yasnygina et al. [2015]. Major oxides were measured by "wet chemistry". Structural setting of the Wudalianchi zone. This zone extends north-south for 230 km at the northern circuit of the Songliao basin, subsided in the Late Mesozoic-Early Cenozoic (Fig. 1). Timing of volcanism and variations of K2O contents in rocks from the Wudalianchi zone. Rocks, dated back to the Pliocene and Quaternary, show the stepwise increasing K2O content interval along the Wudalianchi zone from the southernmost Erkeshan volcanic field (5.6-5.8 wt %) to the northernmost Xiaogulihe-Menlu volcanic field (2.0-9.5 wt %) (Fig. 2). Spatial-temporal clustering of volcanoes in the Wudalianchi field. In terms of the general Quaternary evolution of volcanism in Asia [Rasskazov et al., 2012], spatial-temporal distribution and compositional variations of volcanic products, we distinguish three time intervals of the volcanic evolution: (1) 2.5-2.0 Ma, (2) 1.3-0.8 Ma, and (3) <0.6 Ma. The Central group of volcanoes showed persistent shifting of eruptions from Wohushan (1.33-0.42 Ma) to Bijiashan (0.45-0.28 Ma) to Laoheishan (1720-1721, possibly earlier) to Huoshaoshan (1721) (Figs 3, 4, 5). No spatial-temporal regularity of eruptions in volcanoes of the Erkeshan field and Western and Eastern groups of the Wudalianchi field reflected background activity. Sampling. Representative sampling of rocks from the Wohushan-Huoshaoshan volcanic line was aimed to identify changing geochemical signatures along the whole volcanic line and in the course of eruptions in each volcano (Figs 3, 6, 7). For comparisons, the background volcanoes were also sampled. Silica and alkalis oxides. On the total alkalis-silica (TAS...

Trace-Element and Pb Isotope Evidence on Extracting Sulfides from Potassic Melts beneath Longmenshan and Molabushan Volcanoes, Wudalianchi, Northeast China

“…Pyrolite-normalized incompatible trace-element patterns of rocks from Wudalianchi demonstrate relatively uniform spectra that differ from oceanic basalt signatures [2,30,49]. The rocks showed negative anomalies of Cs, Th-U, Nb-Ta, and peaks of Ba, K, La.…”

“…From a detail sampling [30], volcanic units of cones and related lava flows in the Central group of volcanoes were distinguished in terms of MgO content. The most compositionally variable rocks of the latest eruptions in the Laoheishan volcano were divided into (1) basaltic trachyandesites and phonotephrites (MgO = 7.1-8.2 wt.%), (2) trachyandesites (MgO 5.3-6.3 wt.%), and (3) phonotephrites (MgO = 6.4-6.9 wt.%).…”

“…Symbols are as inFigure 5. The bi-directional dark-blue arrow on (b) shows the advanced generation of a restite signature from the OIB source due to separation of a complementary component (CC) after[30].…”

Origin of Melting Anomalies in the Japan-Baikal Corridor of Asia at the Latest Geodynamic Stage: Evolution From the Mantle Transition Layer and Generation by Lithospheric Transtension

“…It is defined due to specific high-potassic compositions of erupted liquids. The uniqueness of volcanic sources from the Wudalianchi zone for the whole East China was emphasized in multiple papers [Zhang et al, 1995;Chuvashova et al, 2009;Chu et al, 2013;Rasskazov et al, 2016;and other]. The established spatialtemporal variations of rock compositions in the Wudalianchi volcanic field are explained in terms of the magma generation control by the north-south transtension zone in the layer of the lithospheric base that shielded the underlying sub-lithospheric convective mantle from the overlying more enriched lithosphere.…”

“…The eruptions of sub-lithospheric melts from the axial part of the main transtension zone, which took place at 2.5-2.0 Ma, were followed by the propagation of the background liquids from the wider segment of the enriched lithospheric region at 1.3-0.8 Ma. In the past 0.6 Ma, background melting progressed at the margins of the transtension segment simultaneously with local melting along the crack that propagated in the boundary shielding layer under the central part of the background melting region [Rasskazov et al, 2016].…”