Mid-Paleozoic suturing of Proto-Tethys

attributed the presence of batholiths to a former active plate margin. The two granites from the northwest-trending part of the western Kunlun dated by Fan and Wang 1990 as 445 Ma KAr and 480.43 5 Ma UPb belong to this belt. These ages indicate Ordovician subduction. It cannot be ruled out that subduction occurred also during the Cambrian and Silurian. Southward subduction is indicated by the northern position of this older arc granitoid belt within the South Kunlun and its occurrence south of the Oytag-Kudi suture. The interpretation of an Early Paleozoic episode of subduction is difficult to reconcile with coeval granitoid intrusions into oceanic lithosphere, unless some ophiolite obduction occurred relatively early, or unless the upper plate also had an oceanic back-arc region north of the South Kunlun, whose associated oceanic arc was first intruded by more or less M-type? arc granitoids “M- type” sensu Pitcher, 1982 and then obducted?.

5. Mid-Paleozoic suturing of Proto-Tethys

In the Kongur Shan area Mt. Kongur, 7719 m, 200 km westnorthwest of Yecheng, at the transition between the Pamirs and the Kunlun, a slightly metamorphosed succes- sion of shallow marine Ordovician fossil-bearing limestones and arc-related volcanites as well as Ordovician and Silurian clastic deposits occurs above the Sinian strata and below up to 1350 m thick Upper Devonian terrestrial clastic red beds Yao and Hsu¨, 1994, p. 79 and their Fig. 6. A pre-Devonian stratigraphic gap also exists in the western North Kunlun where there is no stratigraphic record between the Sinian rift sequence and unconformably overlying Upper Devonian terrestrial red molasse deposits. The Upper Devonian molasse consists of fluvial and allu- vial fan arkoses and conglomerates as well as volcanogenic strata. At a roadside outcrop, approximately 10 km east- northeast of the Akaz Pass, these clastic rocks are compo- sitionally and texturally immature. Lithic components were eroded from nearby granitoids, rhyolites, acidic to inter- mediate altered pyroclastic rocks and basic volcanites. At the same outcrop we observed an angular unconformity within these sediments with an angle close to the angle of repose. Only a larger angle would justify the interpretation of a tectonic cause for the formation of this unconformity. F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 643 Fig. 8. Map sketch of granitoids of the study area mainly after Matte et al. 1988 supplemented by data from Liu et al. 1988 for the South Kunlun. Note that the northern granitoid belt is much smaller than the southern one. Mazar pluton according to Liu et al. 1988, Gaetani et al. 1991, and own observations. Map depictions of the Kudi pluton differ greatly compare, for example, Liu et al., 1988 and Matte et al., 1996. The anatectic laccoliths after Matte et al. 1996. The thickness of the Devonian molasse may be several hundred meters Matte et al., 1996. Pan et al. 1992 noticed that the Devonian succession exhibits an overall fining-upward trend. The Devonian clastic rocks have been dated by plant fossils reviews by Sengo¨r and Okur- ogullari, 1991; Yao and Hsu¨, 1994. Terrestrial red molasse deposits of Devonian age also occur in the South Kunlun Pan et al., 1992. As shown in Fig. 2, the stratigraphic development of the North and South Kunlun is continuous and similar across the Oytag-Kudi suture as of the Devonian. Thus, the accretion of the South Kunlun against the southern margin of the Tarim Block must have been completed by that time. It appears reasonable to attribute the pre-Devonian stratigraphic gap in both terranes Fig. 2 to uplift and erosion related to regional shortening in the course of accretionary processes. Since shallow marine Ordovician and Silurian strata are preserved in the Kongur Shan area we assume that significant short- ening and uplift started after deposition of the Silurian clas- tic sediments. According to Yao and Hsu¨ 1994, their Fig. 6, uppermost Silurian to mid-Devonian sediments are miss- ing in the Kongur Shan area. This indicates to us that sutur- ing, shortening, and mountain building started during the Late Silurian. Matte et al. 1996 concluded a Silurian colli- sion. They listed a UPb zircon age of 377 Ma and RbSr ages of 392 35 Ma on whole rock, and of 381 4 Ma on biotite on the potassic postkinematic Kudi granite, whose intrusion postdates metamorphism and anatexis in the South Kunlun. This metamorphism and anatexis is indicated by a complex 40 Ar 39 Ar age spectra on K-feldspars of migmatites south of Kudi, suggesting a minmum age of 380–350 Ma, due to Silurian collision Matte et al. 1996. The Devonian molasse of both the North and South Kunlun grades upward into shallow marine Carboniferous and Permian carbonates Fig. 2; Pan et al., 1992 which are well-dated by fusulinids, bivalves, brachiopods and corals De Terra, 1932 as quoted in Matte et al., 1996; BGMR, 1993. The carbonates may be 1 km thick Matte et al., 1996. The fact that these carbonates are marine indicates that the SilurianDevonian Kunlun mountains were already widely denuded when the carbonates accumulated.

6. The Kara-Kunlun accretionary wedge