| Abstrakt: |
Introduction Makran geological province of SE Iran is an east-west trending mountain range related to the Cretaceous-to-recent subduction of the Indian oceanic lithosphere beneath the southeastern edge of the Eurasian plate (McCall, 1997; Saccani et al., 2022). Makran subduction zone from north to south is composed of remnants of Neo-Tethys oceanic crust, and the Late Cretaceous-Miocene sediments called Makran accretionary wedge (Figure 1). Volcanic fields of Shahsavaran, Bazman, West of Khash and Taftan are located north of Makran and have been considered as a subduction related volcanic arc (Dupuy and Dostal, 1978; Moinevaziri, 1985; Biabangard and Moradian, 2008; Saadat and Stern, 2011; Pang et al., 2014). More than 3 kilometers of mainly clastic sedimentary rocks have been subducted beneath the southeastern edge of the Eurasian plate (Kukowsky et al., 2001). Geochemical characteristics of the more primary basalts from the Makran arc indicate that sediments may have caused mantle source enrichment in the Makran subduction zone (Saadat and Stern, 2011). Quantitative approach to the contribution of subducting sediments to the enrichment of the source of basalts in such a voluminous accretionary prism is hard to attain. However, it is assumed here that subduction of sediments is at least one of the enrichment agents of mantle wedge beneath the Makran subduction zone. In this study, chemical composition of Late Cretaceous-Miocene claystones of turbidite suquences from the Fanouj-Bent area are compared with the chemical composition of sediments from other active subduction trenches. They are also deployed to construct binary melt-sediment mixing models in order to evaluate the postulated sediment contribution to the petrogenesis of the Makran basalts. Sedimentary rocks from the Fanouj–Bent area are the oldest among the Makran turbidites, and are thought to be representative of the most viably-recycled sediments in the mantle source of the Makran volcanic arc. Composition of near primary basalts from Makran volcanic arc (MVA) are then compared with the composition of various binary mixtures between a model basaltic melt and sediment-derived melts and fluids. Regional Geology The Late Cretaceous-Eocene (Mohammadi et al, 2016) deep marine turbidite sediments in this area are exposed together with the ophiolitic rocks of the oceanic crust of the Neotethys remnants (Figures 2A and 2B) and limestone (McCall, 1997). The younger Oligocene and Miocene turbidites include thick sequences of sandstone-claystone that were deposited in deep marine, continental slope, and delta environment in the course of evolution of the Makran accretionary wedge (Figures 2C, 2D). Volcanic rocks of MVA are mainly of andesitic and dacitic composition. The Bazman and Taftan stratovolcanoes are composed mainly of dacitic and andesitic pyroclastic rocks and lava flows. Basalts of MVA are: 1) volcanic centers of Shahsavaran which are shields volcanoes composed mainly of thin basaltic lavas (Figures 2E and 2F), 2) numerous monogenic satellite cinder cones scattered around the main Bazman volcano, and 3) cinder cones of west of Kash. Takhte Rostam is a basaltic center located on the southern flank of Taftan volcano. Analytical methods Major and trace element analyses for 2 claystone samples and 6 basaltic samples were performed using X-ray Fluorescence (XRF) spectrometry, and ICP-MS, respectively, at Acme Lab™, Canada. The 6 other claystone samples were analyzed for major and trace elements using XRF and ICP-OES methods in Central Lab of Isfahan University. Three samples of BCR-1 geostandard and three samples of an in-house standard were analyzed simultaneously in both labs, as unknown, to calculate accuracy and precision of the analyses. Table 1 shows geochemical data for the claystone and basaltic samples. Discussion In spidrgrams normalized to continental crust (Figure 7), the claystone samples are enriched in Cs, Rb, Th, U, Ta, Nb, K, Pb, Zr, and Hf and depleted in Ba, and Sr relative to the GLOSS II. These samples seem to be the most enriched sediments among all oceanic trenches of active subduction zones. The quantity of mobile elements in the claystones is comparable to those of the basaltic samples, but the Sr, Ba and Rb amounts are not consonant. Claystone samples are characterized by depletion in Sr (Ave: 171 ppm) and Ba (Ave: 213 ppm), and remarkable enrichment in Rb (Ave: 185 ppm). Compared to claystone samples, the contents of Sr, Ba and Rb in coastal Makran sample (CM) are more comparable to those of the basaltic samples. These similarities suggest that the coastal sediments may have accompanied the Late Cretaceous-Miocene clastic sediments in the mantle enrichment process. The geochemical evidence provided so far for island arc magmas are indicative of the presence and the influence of slab-fluids within and/or from the subducting slabs (Johnson and Plank, 1999; Nakamura and Iwamori, 2009; Schmidt and Poli, 2014; Turner and Langmuir, 2022). Below the solidus temperatures Rb, Sr, Ba, and Pb show more mobility, while at the solidus temperature Th and Be are notably partitioned into the melt rather than fluids (Johnson and Plank, 1999). To evaluate the rate of sediment contribution as a melt (Figure 8) or fluid (Figure 9) in magma generation, two models have been provided in this study. The three end member compositions for calculation of the none-modal batch melting models (Shaw, 1970) are: (1) a near-primary basaltic composition calculated from 15% partial melting of a spinel-lherzolie (MM), (2) average composition of 15% melting of claystone samples (Av.C), and (3) 15% melting of the coastal Makran sample of Jarrard and Lyle (1991). Finally, the binary mixing models between the basaltic MM and the other two end members are compared with the composition of Makran basalts. Th/Yb, Th/Ce, La/Sm, and Sr/Nd ratios are used to show the sediment contribution to the composition of the basaltic samples. Variations of Th/Ce versus Th/Yb ratios and Sm/Yb versus log Th/Yb ratios in basaltic samples show relative consistency with MM-Av.C binary mixing trend (Figure 8). Using these ratios, the sediment melt contribution rate is determined to be up to 10 %. To evaluate the effects of fluids rising from the sediments, in addition to MM, 3 other end members are deployed as: (1) the average composition of 15% fluid released from the claystone samples at 650oC (2) the average composition of 55% fluid released from the claystone samples at 700oC, and (3) 15% fluid released from the coastal Makran sample at 650oC. Binary mixing between MM and about 10% to more than 50% fluid derived from the coastal Makran sample at 650oC (Figure 9), is fairly comparable to the variations of Rb/Nd versus Ba/La and Rb/La versus Sr/Nd ratios. In addition, Elevated Sr/Nd, Ba/La and somehow Ce/Pb ratios are indicative of the contribution of slab derived fluids to the petrogenesis of the basalts. In all calculations, partition coefficients, D values, are taken from Johnson and Plank (1999). [ABSTRACT FROM AUTHOR] |
