Geochemistry and Zircon U-Pb Dates of Felsic-Intermediate Members of the Late Cretaceous Yüksekova Arc Basin: Constraints on the Evolution of the Bitlis–Zagros Branch of Neotethys(Elazig, E Turkey)

During the Late Cretaceous in the Eastern Mediterranean, the northern branch of the southern Neotethys was closed by multiple northward subductions. Of these, the most northerly located subduction created the Baskil continental arc at around 82–84 Ma. The more southerly and intra-oceanic subduction, on the other hand, produced an arc-basin system,the Yüksekova Complex, as early as the late Cenomanian–early Turonian. The abundant and relatively well-studied basaltic rocks of this complex were intruded by dykes, sills and small stocks of felsic–intermediate rocks, not previously studied in detail. The intrusives collected from five different localities in the Elazig region of eastern Turkey are all subalkaline, with low Nb/Y values. Most of them have been chemically classified as rhyodacites/dacites, whereas a small number appear to be andesites. In normal mid-ocean-ridge basalt(N-MORB)-normalised plots, the intrusives are characterised by relative enrichments in Th and La over Nb, Zr, Hf, Ti and high field strength elements(HREEs), indicating their derivation from a subduction-modified source. While their relatively high, positive εN d(i) values(+6.4 and +7.2) might suggest a depleted mantle source for their ultimate origin, somewhat radiogenic Pb values indicate a sedimentary contribution to the source of the rocks. The overall geochemical characteristics indicate their generation in an oceanic arc setting. The zircon U-Pb Laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS) data obtained from five felsic-intermediate rock samples yielded intrusion dates of 80–88 Ma. This suggests that the Elazig oceanic arc-related intrusives are slightly younger than those of the Yüksekova arc-basin system, but coeval with the Baskil continental arc. However, the felsic–intermediate intrusives show different geochemical characteristics(oceanic arc-type, with a lack of crustal contamination)to those of the Baskil continental arc. This indicates that these two igneous systems are unrelated and likely developed in different tectonic settings. This, in turn, supports a geodynamic model in which the northern strand of the southern Neotethys was consumed by multiple northward subductions.

Episodic Paleozoic Adakitic Magmatism in the Eastern Tianshan,Southern Altaids:Implications for Petrogenesis and Geodynamics

The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth,as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins,which are longstanding conundrums for understanding the tectonics of the Paleo-Asian Ocean(PAO).As the processor of the Altaids,the PAO was a long-lived ocean,and its multiple subductions led to substantial continental growth in the Central Asian continent.Here we report our newly discovered Paleozoic adakitic rocks(granite and dacite)developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues.The Jijitaizi granite and dacite contain high concentrations of Sr(203-343 ppm)and low concentrations of Y(3.00-6.36 ppm)and Yb(0.23-0.62 ppm),indicative of the adakitic affinity.Zircon U-Pb dating results show that the adakitic rocks were formed in three episodes at 372.26±1.51 Ma,356.02±1.11 Ma and 334.51±0.68 Ma from the Late Devonian to Carboniferous.The rocks are depleted in Nb,P and Ti,and possess variable Mg#values(28.51-57.22),high Na_(2)O/K_(2)O ratios(1.39-4.94),as well as relatively depleted Hf isotopic compositions,all of which point to a subduction origin from slab melting.Our new finding with the date from the Jijitaizi complex(ca.321 Ma)suggests continuing subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous.Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca.372 to 321 Ma.