Petrogenesis of island-arc volcanic rocks from the Char suture-shear zone (East Kazakhstan)
Kurganskaya E.V., Safonova I.Y., Simonov V.A.
V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia
e.kurganskaya@gmail.com
The Char suture-shear zone (SSZ) of East Kazakhstan is an important segment of the Central Asian Orogenic Belt. It is special for an extremely complicated tectonic structure and co-occurrence of ultramafic-mafic ophiolitic units, various volcano-sedimentary sequences, and Paleozoic basaltic lavas [2,3]. Its study is important for reconstructing the geodynamic history of the western parts of the CAOB including vast folded areas in East Kazakhstan, SW Siberia, NE China and western Mongolia. The main feature of the Char SSZ is the Char opiholitic belt hosting Late Devonian-Early Carboniferous oceanic basalts [6] and located in its central axial part. Previously, Belyaev [2] mentioned andesitic to dacitic volcanic rocks east of the Char suture. During the 2008-2009 field works we found volcanic and subvolcanic rocks of possibly suprasubduction origin west of the Char suture. In this paper, we present first geochemical data on these volcanics.
The rocks under study are tholeiitic to calc-alkaline basalts, andesibasalts and andesites. The proportions of FeO, MgO and TiO2 suggest two magma series: higher-Fe tholeiitic (SiO2 = 52.3; TiO2 = 0.58; #Mg = 38; CaO/Al2O3=0.5) and lower-Fe calc-alkaline SiO2 = 54.5; TiO2 = 1.1; #Mg = 51; CaO/Al2O3=0.3). The tholeiitic varieties are less enriched in the LREE, then the calc-alkaline ones and have lower Nb and Th (La/YbN = 1.1 and 3.0; Nb = 0.65 and 3.93; Th = 0.54 and 1.86, respectively). One sample (Ch-59-08) is a high-Mg and low-Ti andesibasalt, which is characterized by higher Ni, Cr, Zn, Al2O3, lower LREE, Sr, P, Y, Nb, and Th compared to the tholeiites and calc-alkaline varieties. Preliminary, we regard it boninite. Compositionally, for both major and trace elements, the bulk calc-alkaline and tholeiitic rocks are similar to the melt inclusions from their hosted clinopyroxenes. Simonov et al. [7] showed that the composition of melt inclusions suggests that magmatism evolved from primitive island arc with boninites to mature island arc with calc-alkaline melts. Their trace element composition is also close to island-arc tholeiitic and calc-alkaline series. The parental melts crystallized at 1150–1190°C. Numerical simulation and in-situ ion-microprobe analysis indicate that the melts contained up to 1 and 0.84 wt % water, respectively. The calculated liquidus temperatures are consistent with the temperatures of homogenization of melt inclusions. The calculations based on melt inclusion composition showed that the primary melts of the Char island-arc basalts were produced at 1350–1530°C temperatures and 50–95 km depths, i.e. close to the parameters of melting of typical Pacific tholeiitic and boninitic island-arc magmas. Thus, the compositions of whole-rock samples and melt inclusions in clinopyroxene suggest their formation in an active continental margin setting, including tholeiitic and calc-alkaline island-arc systems [7].
In the binary diagrams MgO is positively correlated with FeO and CaO and negatively with SiO2, Al2O3 and TiO2 suggesting fractionation of plagioclase and clinopyroxene and, to a lesser degree, titanomagnetite. For petrogenetic implications we performed modeling of clinopyroxene fractional crystallization and melting based on the mantle source parameters and approaches discussed in Pfander et al. [5]. For clinopyroxene fractionation modeling in the Nb versus Zr system we chose two samples, basalt (C-21b-08) and andesibasalt (Ch-59-08), which are characterized by lowest contents of Nb and highest MgO. Our results showed that in most of the samples clinopyroxene fractionation did not notably affect the relationships in the Nb versus Zr/Nb system. Two samples, Ch-69-08 and Сh-44-09, could be derived from the melt, which produced high-Mg basalt C-21b-08, due to clinopyroxene fractionation.
Fig. Main geochemical features of Char volcanic rocks. A – Al2O3-FeO*+TiO2-MgO classification diagram [4]; tholeiitic series: TA – andesite, TD –dacite, TR – rhyolite; calc-alkaline series: CB – basalt, CA – andesite, CD – dacite, CR– rhyolite; kom – komatiite. B - primitive mantle-normalized multi-component trace element patterns; the normalization values are from Sun and McDonough [8].
In addition, we checked the fractionation of pyroxene as a probable factor controlling the content of Ti (Ti versus Nb system) and found that the observed variations of Ti concentrations in several samples can be achieved by fractionation of 10 to 30% of pyroxene. For the calculations we used a Nb-depleted MORB-type melt and a hypothetical melt compositionally similar to the most Ti-Nb-depleted sample Ch-09-08. Such fractionation could be possible in a shallower magma chamber.
For melting modeling we used the Nb–Yb systematics to calculate the composition of melts produced by different degrees of melting and variable source compositions, applying the equation for nonmodal batch melting [1]. We tested the melting of primitive (garnet peridotite, spinel lherzolite) and moderately and high-Nb-depleted mantle sources (depleted harzburgites), which could probably produce the basaltic melts ([6] and references therein). The modeling suggests that most Char volcanics formed from the melts derived from depleted harzburgite.
Thus, for the first time we identified volcanic rocks with suprasubduction chemical characteristics which occur as tectonic sheets in the western part of the Char ophiolite belt. The volcanic rocks are tholeiitic and calc-alkaline basalts to andesites. They are characterized by nearly flat REE patterns and Nb negative peaks in the multi-element diagrams. According to petrological data and geochemical modeling the melts, which produced the Char volcanic rocks, were derived from a strongly depleted mantle source (sub-arc harzbirgite), experienced fractionation of clinopyroxene and crystallized at relatively high temperatures.
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