Oytag-Kudi suture and Kudi Ophiolite Complex

The alternation of metacarbonates and metatholeiites indicates that marine conditions persisted during the rift process. Therefore, the visited area of the North Kunlun was probably not located on a rift dome, as this would have likely caused uplift of the platform above sea-level. The Sinian rift sequence of the Akaz Pass area might have been located to the side i.e. “north”, according to present day directions of such a dome. This interpretation implies a dip-slip type of rifting. Alternatively, one could argue that rifting was due to strike-slip motion as this would neither require nor induce a rift dome Mattern et al., 1998. However, strike-slip rift- ing appears to be an unlikely option because of the abun- dance of volcanogenic strata in the Sinian succession, considering that strike-slip rifts are low-volcanicity rifts at best Mattern et al., 1998.

3. Oytag-Kudi suture and Kudi Ophiolite Complex

The Oytag-Kudi suture which separates the North and South Kunlun is marked by an alignment of ophiolite slices which appear narrow in map view Fig. 1; Liu et al., 1988. The suture is inclined towards the South Kunlun Pan et al., 1992, their Fig. A-2; Matte et al., 1996, their Fig. 2. Little is known about the geology of this suture trace “suture trace” sensu Sengo¨r et al., 1988, their Fig. 21. Yang et al. 1996 reported peridotite, cumulate gabbro and basalt from the Kegang ophiolite occurrence ca. 100 km NW of Kudi. Approximately 80 km east of Kudi, the structure along the suture trace appears to be complicated, in so far as there is a large lenticular, possibly fault-bounded rock unit, adjacent to the suture north of the ophiolites, which displays an en e´chelon rock fabric Matte et al., 1996. In Figs. 1 and 8 we assign it to the South Kunlun. Knowledge of the Kudi Ophiolite Complex, which appears much larger in map view than the ophiolite units of the suture trace, is much more detailed. The Kudi Ophio- lite Complex represents an obducted ophiolite unit which was thrust from the suture and emplaced on the South Kunlun Fig. 3. We studied the northern and eastern part of this complex in the northsouth-trending Kudi Valley along the road north of the village of Kudi and in the small, canyon-like Yishak Valley which trends approxi- mately eastwest and leads into the Kudi Valley from the west Fig. 3. We investigated the southern part of the complex at the northern slope of the Boziwan Valley which joins the Kudi Valley from the west at the northern margin of Kudi Fig. 3. Except for a sheeted dike complex F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 639 Fig. 2. Juxtaposition of the North and South Kunlun’s stratigraphy. Note the similar development as of the Devonian. Fig. 3. Cross section sketch of the obducted Kudi Ophiolite Complex, constructed mainly from the investigations in the Boziwan Valley and Kudi Valley road section. Fault marked with “x” is the one depicted on Fig. 6, ca. 15.5 km north of Kudi at the bridge. Fold vergence close to the Yishak Valley may be due to post-obductional shortening. Minor outcrops of metamorphic rocks are not shown. we found all main layers of a complete ophiolite section Fig. 4. Instead of the sheeted dike complex we observed a thick layer of massive basalt. Yang et al. 1996, who investigated the Kudi Ophiolite Complex as well as some ophiolites of the suture trace, also noted the absence of sheeted dikes. A summary of our findings pertaining to the layering of the complex is shown in Fig. 4. The Dunite as the lowermost layer, generally exhibits a coarse-grained, structureless fabric. Locally we observed thin parallel laminations of chromite which are intersected by only a few centimeters thick dikes of pyroxenite. More- over, we found up to 1.5 m thick hornblendite dikes in the dunite. The peridotite above the dunite is partly serpenti- nized. Among the rocks of this layer we distinguished two varieties — a relatively fine-grained and massive one, and another that displays layering of pyroxene cumulates. Within this layer also distinct pyroxenite bodies occur. The lithology of the peridotites is dominated by harzburgite Yang et al., 1996. According to Yang et al. 1996, the peridotites are highly depleted. The next layer consists of coarse-grained gabbro. We can neither describe the transition from the gabbro layer to the next layer, nor do we know whether a transition even exists. The next layer is thick and comprises a gener- ally fine-grained, massive basalt Fig. 4 above which a layer of pillow basalt accumulated. The diameter of the pillows usually does not exceed 1 m Fig. 5. The pillows display glassy rims and amygdales. According to Pan et al. 1992, the pillow basalts of the Oytag-Kudi suture repre- sent mature ocean ridge type tholeiites, but, according to Yang et al. 1996, lavas from this suture zone display geochemical patterns suggesting a supra-subduction zone environment. Above the pillow basalts there are lenses of hematitic chert up to 2 cm thick, shales and tuffites. The following layer is represented by massive, amygdaloidal, slightly red or brown basalt. A debrite with a thickness of several deci- meters is associated with this layer in the Yishak Valley. At the base of the uppermost layer, lenses of up to 1 dm thick hematitic chert occur. The bulk of the uppermost layer, however, comprises red shales and green tuffites which may alternate in thin layers of only a few centimeters or form monotonous successions of several meters or even tens of meters. The red shales are frequently intercalated with gray graded layers of several millimeters to centimeters in thick- ness. The grain size of the tuffites ranges from dust to lapilli. Coarse tuffites from the Yishak Valley bear lithic compo- nents of basic volcanites, devitrified glass components, plagioclase phenocrysts and fragments displaying flow fabrics, like deformed vesicles. All of these rock types appear to be silicified. The ratio between the amount of suture sediments and basic to ultrabasic oceanic igneous rocks is relatively small in the Kudi ophiolite complex. According to map analyses e.g. Liu et al., 1988, this also holds true for the suture trace. Age information pertaining to the Oytag-Kudi ophiolites is controversial. Moreover, radiometric literature informa- tion is often unspecific about the applied method, investi- gated minerals, sample location, type of age and so forth. Rb–Sr isochron dating of basaltic rocks by Jiang et al. 1992, as quoted by Yang et al., 1996 yielded an age of 360 Ma. According to the Institute of Geology and Mineral Resources, Urumqi, Xinjiang correspondence, radiolarian cherts of the Kudi ophiolites date as Carboniferous to Permian. Wang 1983 reported an age of “earlier than 860.5 Ma” for an ultrabasic rock. He also reported the age of a quartz diorite which intruded volcanic rocks of the Kudi F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 640 Fig. 4. Layering of the Kudi Ophiolite Complex. The total thickness measures more than 2 km Wang, 1983. According to Yang et al. 1996, arc-typical lavas are a part of the complex. Fig. 5. Roadside outcrop of pillow lava belonging to the Kudi Ophiolite Complex, ca. 8 km north of Kudi. Ophiolite Complex to be 517 Ma. Pan et al. 1992 mentioned a Rb–Sr isochron age of 816 Ma for a pegmatitic amphibolitic dike which intruded an ultramafic rock, and a K–Ar whole rock age of 517 Ma for a diorite which intruded volcanites north of Kudi. We wonder whether Pan et al. 1992 were referring to Wang’s 1983 data and whether the different numbers “860” of Wang 1983 and “816” of Pan et al. 1992 resulted from a translation mistake or mix- up in one of the two English texts as both numbers are phonetically very similar. According to Pan et al. 1992, this diorite was also dated as 458 Ma Rb–Sr isochron and 480 Ma 40 Ar 39 Ar. They also reported ages pertaining to a granite which intruded the volcanites of 384 Ma 40 Ar 39 Ar and 423 Ma Rb–Sr isochron. Pan et al. 1992 also listed a model age for pillow lavas, not located in the Kudi area, of 900–600 Ma. Xu et al. 1992 determined the following ages for a granodiorite which intruded the pillow lava: 474 Ma 40 Ar 39 Ar plateau age on hornblende, 449 24 Ma 40 Ar 39 Ar– 39 Ar 36 Ar isochron age on biotite, and 458 Ma UPb concordant age on zircon. The same age numbers partly different methods were also published by Matte et al. 1996 for a schistose granodiorite south of Akaz Pass which intruded mafic rocks. Its location is apparently indi- cated on their Fig. 2 with the age information “gd4, 460 Ma”. We had the opportunity to investigate the zone between the southernmost exposures of this pluton and the northern- most part of the Kudi Ophiolite Complex, at the bridge approximately 15.5 km north of Kudi. Microscopic kine- matic evidence shows that the granodiorite was ductilely sheared in a strike-slip mode along the northwestsoutheast directions Mattern et al., 1996. The main mylonitic folia- tion of the granodiorite is conspicuous in the field Fig. 6. We also observed steeply dipping brittle faults Mattern et al., 1996, p. 708–709, trending eastwest Fig. 7. The contact between the Kudi Ophiolite Complex and the pluton is covered by a 300 m wide zone of unconsolidated sedi- ments Fig. 6. The northernmost outcrop of the Kudi Ophiolite Complex also exhibits signs of brittle fracturing. The massive basalt was intruded by a mafic dike. We did not find any evidence for an intrusive plutonophiolite contact. Instead we interpret the contact relation between the pluton and the Kudi Ophiolite Complex as a faulted one Fig. 3. Although we saw many outcrops of the Kudi Ophiolite Complex in the three valleys, we found no evidence of granitoid intrusions into the complex. All contacts between granitoids and the ophiolites were covered by unconsoli- dated sediment. Within the ophiolite complex, we also did not observe any aplite or silicic pegmatite dikes which could be associated with the granitoid intrusions. The literature provides neither specific phenomenological nor formal location details on the intrusive contact relations between granitoids and ophiolites. Acknowledging the controversial age data, Pan et al. F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 641 Fig. 6. Southern margin of the 460 Ma granodiorite, 15.5 km north of Kudi. In the foreground on the right side, the pluton displays a mylonitic foliation 015 85, marked by the arrow on the lower right. The view is exactly parallel to the foliation towards 285 8. Note that in the projected extension of the foreground mylonitic zone, the mountain crest of the background, displays a saddle upper arrow. Note the more or less horizontally stratified unconsolidated river deposits in the south and the loess in the center covering the contact between the pluton and the more southerly located Kudi Ophiolite Complex. 1992 and Mattern et al. 1996 considered the ophiolites of the Oytag-Kudi suture to have formed in the Proto-Tethys between the Sinian and Early Paleozoic. Further time constraints on the formation of these rocks is provided by the age of subduction-related magmatites see below which formed during subduction of the Proto-Tethys Ocean since oceanic lithosphere must first be created before it can be subducted and since ocean-spreading can be coeval with subduction.

4. Paleozoic subduction of Proto-Tethys beneath the South Kunlun