Late Paleozoic to Early Mesozoic subduction of the Paleo-Tethys Ocean beneath the South Kunlun

and volcanites, embedded in a pervasively sheared, cleaved matrix of sericite-quartz schist, chlorite-sericite schist, two- mica schist and phyllite. Since the maps by Liu et al. 1988, BGMR 1993, and Matte et al. 1996 do not indicate ophiolites in the Mazar area, and since Yao and Hsu¨ 1994 were unspecific about the location of the studied ophiolite outcrops, we are unable to show them in Fig. 1. Another ophiolite unit occurs east of Dahongliutan Fig. 1. Also farther east, outside the study area, there is an ophiolite body at Mt. Muztag 7723 m, 700 km eastsoutheast of Hotan — not to be confused with Mt. Muztag, 7282 m, 115 km southsoutheast of Hotan; Pan et al., 1992; Yao and Hsu¨, 1994, their Fig. 8. The presence of ophiolites in rocks of the Kara-Kunlun Liu et al., 1988; Yao and Hsu¨, 1994 is impor- tant because it indicates that internal thrust planes exist in the siliciclastic Kara-Kunlun belt, such as one would expect to occur in an accretionary wedge. The ratio between the amount of suture sediments and basic to ultrabasic oceanic igneous rocks is relatively large in the Kara-Kunlun wedge. In the westernmost part of the Kara-Kunlun, the clastic rocks of the Kara-Kunlun are either unmetamorphosed or display a very low metamorphic grade Gaetani et al., 1990. According to Matte et al. 1996, the clastic rocks are affected by only low-grade metamorphism. At several road- side outcrops between Mazar and Hez Pass and also farther east, for example, at the Qitai Pass, unmetamorphosed strata can be observed. At several places we found evidence of contact metamorphism. These areas include the aureole of the Mazar pluton, west of Mazar, the Dahongliutan area, and a location 40 km eastsoutheast of Shanshilli outcrop along a new road, Mattern et al., 1996. At the latter we found sillimanite-bearing metashales. Ductile faults can be observed in these zones below. We noticed that the strata of the Kara-Kunlun mainly dip to the north and northnortheast, that is inclined towards the South Kunlun. Observations of the preferred dip include those that were made on mountain-scale outcrops, for exam- ple, in the areas around the Hez Pass, Dahongliutan, and the Qitai Pass. Folds exhibit a south or southsouthwest vergence. The observed slaty cleavage is genetically related to the folds Mattern et al., 1996. These aspects are compa- tible with the interpretation of an accretionary wedge which formed south of the South Kunlun in response to northward B-type subduction. As shown below, the Kara-Kunlun was intruded by subduction-related granitoids indicating an active margin environment in which the Kara-Kunlun wedge became the site of arc magmatism. Whereas the stratigraphy and age of the Kara-Kunlun’s rocks are poorly understood, the geodynamic significance of the Kara-Kunlun zone appears to be clear. Hsu¨ 1988 inter- preted the Kara-Kunlun area as an accretionary wedge which formed due to northward subduction of the Paleo- Tethys Ocean beneath the South Kunlun. This view is consistent with the flyschoid facies and composition of the thick clastic sediments and their association with tuffites, ophiolite me´langes and arc granitoids as well as with the implied internal thrust planes and with the dominant dip direction of strata and the observed vergences. The Kara- Kunlun accretionary wedge can be correlated with the Sonpan Ganze Belt or parts of it Bayan Har Group farther east Sengo¨r and Okurogullari, 1991; Matte et al., 1996; Mattern et al., 1996. The Songpan Ganze Belt occupies a position south of the Kunlun, like the Kara-Kunlun accre- tionary wedge, and is also characterized by a great thickness and by a dominance of fine-grained siliciclastic deposits Leeder et al., 1988; Coward et al., 1988; Nie et al., 1994. It has also been interpreted as an accretionary wedge south of the eastern Kunlun Leeder et al., 1988.

7. The Uygur Terrane and adjacent sutures

The Uygur Terrane located south of the Kara-Kunlun accretionary wedge Fig. 1 is the southernmost unit we inspected. Although the Uygur Terrane is a poorly under- stood element of the regional tectonic collage there is certainty that its rock facies contrasts sharply with that of the Kara-Kunlun accretionary wedge. No stratigraphic rela- tionship exists between these two units. We observed a thrust contact between the two terranes in the Tianshuihai area Mattern et al., 1996. The Uygur Terrane consists of a fossiliferous and generally well-dated Paleozoic to Triassic shallow marine succession BGMR, 1993. The succession measures hundreds of meters in thickness and contains a significant amount of carbonates. Because we could only inspect a few of the formations within the terrane we decided not to depict its stratigraphic development. To the south, the Uygur Terrane is separated from the south- erly located KarakorumQiangtang Terrane by the “cryptic” Longmu Co suture Fig. 1 of Baud 1989, which is also referred to as the “Taaxi-Qiaoertianshan-Hongshanhanu suture” Pan et al., 1992. Since we did not reach this suture, the following information is taken from the literature. Although most of this suture has been covered by post-accre- tionary deposits Pan et al., 1992 ranging from the mid-Juras- sic to the Quaternary Liu et al., 1988; BGMR, 1993, Pan et al. 1992 identified this zone as a suture because it delineates blocks of marked geological and paleontological differences. Ophiolites do not occur in the suture segment south of the western Kunlun but are known from this suture farther east Pan et al., 1992. This zone also represents the northern boundary for the distribution of marine Jurassic strata Pan et al., 1992. There is a mid-Jurassic suture overlap assemblage whose age is documented by a brachiopod fauna BGMR, 1993, indicating that this segment of the Paleo-Tethys was eliminated at the latest by the mid-Jurassic.

8. Late Paleozoic to Early Mesozoic subduction of the Paleo-Tethys Ocean beneath the South Kunlun

In the western Kunlun there is no rift sequence preserved which could help to determine when the Paleo-Tethys F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 645 Ocean formed to the south of the South Kunlun. However, there are arc magmatites whose ages indicate at least when melts developed in response to subduction of Paleo-Tethys lithosphere. The onset of subduction must have predated arc magma generation, and ocean spreading must have predated, and may have been coeval with, subduction. From a Kunlun perspective one must conclude that ocean spreading of the Paleo-Tethys must have started before the Late Carboniferous. Upper Carboniferous and Permian intermediate and basic volcanites intercalated with the well-dated Upper Paleozoic shallow marine carbonates of the South Kunlun Fig. 9 are of the calc-alkaline arc type Pan et al., 1992. They repre- sent the earliest evidence for a new subduction cycle. Further evidence is provided by Late Paleozoic to Early Mesozoic orogen-parallel arc batholith belts Fig. 8. As mentioned above, Zhang et al. 1992 distinguished an older Paleozoic arc granitoid belt in the north and a younger Late Paleozoic to Mesozoic one in the south of the South Kunlun. In this chapter we are concerned with the southern belt which is significantly larger than the northern one Fig. 8. The finding by Zhang et al. 1992 is based on radio- metric and geochemical evidence as well as on considera- tions of the geological setting. An altered granodiorite at Xaidulla yielded a RbSr isochron age of 267 Ma on biotite Xu et al. 1992. From a granite located 22 km northeast of Mazar an 40 Ar 39 Ar– 39 Ar 36 Ar isochron age of 211.8 10.8 Ma was published by Xu et al. 1992. For the same pluton an 40 Ar 39 Ar whole rock age of 211 8 Ma and an 40 Ar 39 Ar minimum age of 180 10 Ma on K-feldspars was reported sources in Matte et al., 1996. A red porphyry volcanite located close to the granite yielded a RbSr whole rock age of 180 10 Ma source in Matte et al., 1996. According to Pan et al. 1992, the plutonic rocks of the South Kunlun belong to the calc-alkaline series and owe their genesis to subduction and related arc magmatism. The Kara-Kunlun accretionary wedge was also affected by this magmatic arc activity Matte et al., 1996. Mesozoic arc batholiths intruded the Kara-Kunlun sediments, display- ing a trend parallel to the orogen. Gaetani et al. 1991 obtained a KAr biotite cooling age of 171.5 5.4 Ma for the Mazar Pluton located west of Mazar. Xu et al. 1992 determined an 40 Ar 39 Ar plateau age of 184 Ma on biotite and a low intercept age on zircon of 199.3 1 2.222.5 for the Mazar monzogranite. Xu et al. 1992 also determined an 40 Ar 39 Ar plateau age of 187 Ma for a granite which belongs to the batholith south of the road between Shanshilli and Kengxiwar. They also published a lower intercept age on zircon from a granite near Kengxiwar of 192 Ma. More- over, they dated a monzogranite located 30 km southeast of Kengxiwar? 215 Ma 40 Ar 39 Ar, biotite and 196.3 Ma 40 Ar 39 Ar plateau age, biotite. Matte et al. 1996 carried out 40 Ar 39 Ar measurements on muscovites and biotites from granites along the Karakax Valley area south of Shan- shilli and Kengxiwar and at Dahongliutan yielding plateau ages of 190 8 Ma and 177 3 Ma. The Kara-Kunlun accretionary wedge or part of it must have already formed before the intrusion of those granitoids with the Upper Triassic ages. Matte et al. 1996 and Mattern et al. 1996 concluded that the magmatites of the southern South Kunlun and the Kara-Kunlun formed as a result of the subduction of Paleo- Tethys at a north-dipping Benioff plane beneath the South Kunlun. Apparently, arc magmatism started to affect the South Kunlun already during the Late Carboniferous volca- nites and the Kara-Kunlun only as of the Late Triassic. In both areas, arc magmatism seems to have ceased during the mid-Jurassic. In Kara-Kunlun, arc activity could have slightly outlasted that in South Kunlun. Mattern et al. 1996 hypothesized that with the growth of the Kara- Kunlun accretionary wedge, the trench might have moved oceanwards and with it the magmatic front, so that the Kara- Kunlun became the site of arc granitoid intrusion. The subduction polarity is indicated by the geometry within the KunlunKara-Kunlun subduction accretion zone with the early magmatic arc in the northerly located South Kunlun and the Kara-Kunlun accretionary wedge to the south. The observed vergences in the Kara-Kunlun wedge lend further evidence for a north-dipping Benioff plane. One of our goals during fieldwork was to find and kine- matically analyze ductile shear zones within arc batholiths or within their contact aureoles. The kinematic data obtained by Mattern et al. 1996 are interpreted here as clues as to the kind of plate convergence. We reason that arc magmatism is concurrent with subduction, and that deformation observed within the “still hot” arc rocks and adjacent country rocks should, therefore, have recorded information of the tectonic regime controlled by the kind of plate convergence. We found ductile faults in the vicinity of, and with a trend parallel to, the South KunlunKara- Kunlun boundary at Xaidulla in the southern South Kunlun and 40 km eastsoutheast of Shanshilli in the northern Kara- Kunlun outcrop along a new road, Mattern et al., 1996. The distance between both locations amounts to 55 km. Mylonites of these faults were sampled and analyzed. In both cases we found microscopic evidence for dextral strike-slip motion Mattern et al., 1996. At the first location, the heat required for ductile shearing was provided by the previously mentioned altered Xaidulla granodiorite dated 267 Ma Xu et al., 1992. According to the map by Matte et al. 1996, this granodiorite belongs to a batholith from which the also mentioned granite located 22 km northeast of Mazar was dated 211 10.8 Ma 40 Ar 39 Ar– 39 Ar 36 Ar isochron by Xu et al. 1992. For the same granite the above listed ages of 211 8 Ma 40 Ar 39 Ar whole rock and 180 10 Ma 40 Ar 39 Ar mini- mum age were determined sources in Matte et al., 1996. It has to be noted that the granite of the Mazar area is located at a distance of 80 km west of Xaidulla. The ages range from the Upper Permian to the uppermost Triassic to the mid- Jurassic minimum age. We are inclined to interpret these F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 646 data as an indication that ductile deformation conditions have existed at Xaidulla during the Triassic and during mid-Jurassic cooling. The granitoid which provided the heat for mylonitization of contact-metamorphic sillimanite-bearing shales at the second location belongs to the batholith south of the road between Shanshilli and Kengxiwar, from which Xu et al. 1992 determined the above mentioned 40 Ar 39 Ar plateau age of 187 Ma and the lower intercept age on zircon of 192 Ma. These are Early Jurassic ages. We assume that ductile deformation conditions may have existed during the Early Jurassic and possibly lasted to the mid-Jurassic during the cooling process. If the finding of dextral ductile faults does indeed reflect the kinematic relation between the overriding and the down- going plate, plate convergence should have been of dextral obliquity during the Early Mesozoic.

9. Mesozoic suturing of Paleo-Tethys