Geologic Framework
The geologic framework of the Qinling orogen was built up through interplay of three blocks, the North China block (including the North Qinling), the South Qinling, and the South China block, separated by the Shangdan and Mianlue sutures (Fig. 2). The Shangdan suture resulted from Middle Paleozoic collision of the North China block and the South Qinling. The Mianlue suture resulted from Late Triassic collision of the South Qinling and the South China block. Present upper crust of the Qinling is structured dominantly by thrust¯fold systems. The North Qinling displays thick-skinned deformation with crystalline basement involved, whilst the South Qinling is characterized by thin-skinned thrusts and folds detached above the Lower Sinian. Two types of Precambrian basement, crystalline and transitional, are defined according to lithology and metamorphic grade and different in age. Stratigraphic and sedimentary architecture is characterized by distinct zonation.
Fig. 2. Generalized geologic map of the Qinling, showing its regional tectonic setting, internal divisions and distribution of two types of basement. Numbers in circles indicate amphibolite and granulite facies assemblages comprising crystalline basement: 1=Qinling Group; 2=Tongbai Group; 3=Douling Group; 4=Xiaomoling Group; 5=Foping Group. Numbers in squares represent greenschist or low-amphibolite facies assemblages making up transitional basement: 1=Kuanping Group; 2=Shuixian Group; 3=Wudang Group; 4=Yunxi Group and Yiaolinghe Group; SS=Shangdan suture; MS=Mianlue suture; MF=Machaoying Fault; LMS=Longmenshan orogen ( Meng et al., 2000).
2.1 Upper crustal structures
Present-day topographic expression of the Qinling Mountains is mainly the result of Late Mesozoic¯Cenozoic tectonics, but its fundamental geologic framework has been formed through long-term interplay among diverse blocks. In map view, the Qinling orogen is divided into the North and South Qinling, separated by the Shangdan suture (Fig. 2). The northern border is marked by a relatively narrow, straight, and steep north-dipping fault zone, the Machaoying fault zone, which is a normal fault controlling the Cenozoic rifted basin to the north (Fig. 3). The Mianlue suture zone, greatly modified by Late Mesozoic thrusting, marks the strongly curved southern border. The South Qinling contains different terranes that consist of distinct lithology and have different tectonic histories (Fig. 2). Phanerozoic sedimentary successions are well preserved in the Xunyang and Liuba terranes, whereas the Tongbai¯Dabie, Wudang, and Foping terranes are made up of metamorphic rock assemblages, including the well-known UHP rocks.
Fig. 3. Three geologic cross-sections across the Qinling orogen. Note that the South Qinling is dominated by a thin-skinned thrust system detached above the Lower Sinian, whilst the North Qinling displays thick-skinned style. The Mianlue ophiolitic complex might be covered by thrust sheets in most localities along the southern boundary (sections b¯b' and c¯c'). NCB=North China block; SCB=South China block; VS=vertical scale; HS=horizontal scale (Meng et al., 2000).
The Qinling upper crust is dominantly structured by thrust¯fold systems, thin-skinned style in the South Qinling and thick-skinned in the North Qinling (Fig. 3). The thin-skinned deformation is characterized by south-vergent thrusts and folds in the Xunyang and Wudang terranes (sections b¯b' and c¯c' in Fig. 3) and the sole thrusts are detached above the Lower Sinian in most cases . The thrusts in the Liuba terrane, however, display overall northward vergence (section a¯a' in Fig. 3). Variation in vergence is possibly due to distinct conditions on the southern boundary ( Zhang et al., 1995). The Liuba terrane is confined on the south by the Precambrian basement uplift (the Bikou terrane), which acted as a buttress and resulted in the northward thrusting. In contrast, the Xunyang and Wudang terranes are bordered on the south with sedimentary covers of the South China block, and thus the thrust¯fold system could propagate with consistent southward vergence over the South China block in Late Mesozoic time. Roughly along the 108° longitude, there exists a north¯south-trending wrench fault zone, which might serve as a transfer fault, accommodating oppositely-directed propagation of thrust¯fold systems on its eastern and western sides ( Zhang et al., 1995).
2.2 Precambrian basements
Two types of Qinling Precambrian basement, crystalline and transitional, are distinguished primarily on the basis of lithology and metamorphic grade (Fig. 2). The crystalline basement is Late Archean to Paleoproterozoic in age, and apparently went through multiphase deformation, high-grade metamorphism and intense migmatization ( Zhang, 1988). It is composed largely of amphibolite and granulite facies assemblages, such as biotite plagiogneiss, two feldspar granulites, and graphic marbles, as represented by the Qinling Group and Douling Group. It is the main component of the Foping and Tongbai¯Dabie metamorphic terranes.
In contrast, the transitional basement is of Mesoproterozoic age, as represented by the Kuanping Group in the North Qinling and the Wudang Group and the Yaolinghe Group in the South Qinling. It consists of low-grade metamorphic rock assemblages, such as greenschists and amphibolites, and is internally structured by pervasive foliations and recumbent isoclinal folds. These features distinguish the transitional basement from both the Archean crystalline basement and the overlying Late Neoproterozoic sedimentary covers in composition, deformation style, and metamorphic grade. The main lithology of the basement is metavolcanics that are bimodal and alkaline in nature, enriched in REE, and dated at 1600¯1000 Ma (Zhang et al., 1995). These facts suggest that they, together with associated sediments, were products of continental rifts of Mesoproterozoic age. Early Neoproterozoic (from 1000 to 800 Ma) continental convergence, or the Jinning Orogeny, resulted in deformation and metamorphism of these volcanites and sediments (Gao et al., 1996), thus forming this transitional basement.
2.3 Stratigraphy and sedimentology
The North and South Qinling display differing stratigraphic¯sedimentary sequences from Late Neoproterozoic to Early Triassic (Fig. 4). The Sinian tillites and overlying platform carbonates (dolomites) were deposited over both the South Qinling and the South China block. These facies associations contrast with their equivalents in the North China block, where conglomerates and sandstones are dominant. Contemporaneous sediments are absent in the North Qinling.
Fig. 4. Schematic diagram showing stratigraphic framework of the Qinling and
adjacent regions. Refer to Fig. 2 for location of subzones
in the South Qinling (Meng et al., 2000).
Early Paleozoic sediments change from shallow-marine in the North China block to deep-marine facies in the North Qinling. The southern North Qinling is occupied by a late Early Paleozoic volcanic massif of island-arc affinity, the Danfeng arc massif , which resulted from northward subduction (present orientation) of a paleo-ocean between the North and South Qinling ( Zhang, 1988). Sedimentary sequences south of the massif consist of turbiditic sandstones with abundant pyroclastics in the lower part and shelf-deltaic siliciclastics in the upper. The sequences are of Ordovician to Silurian ages , synchronous with growth of the Danfeng arc massif, and the pyroclastics are also shown derived from the massif . So the sediments south of the volcanic massif are now interpreted to be forearc fills ( Meng et al., 1997). The equivalent strata in the South Qinling and the South China block, however, are dominated by shelf carbonate and siliciclastic facies.
Middle¯Upper Paleozoic successions vary greatly in both stratigraphy and sedimentation within the South Qinling (Fig. 4). The Silurian is only present in middle and southern subzones, and consists predominantly of deep-marine siliciclastics and turbidites . In the northern subzone, the Devonian marine sandstones transgressively overlie the underlying strata and pass up into Carboniferous continental deposits. Shallow-marine siliciclastics and carbonates were deposited in the middle subzone from Devonian to Early Triassic times. Equivalent strata are missing in the northern part of the southern subzone. This absence may result from uplifting and unroofing induced by the Late Triassic collision along the Mianlue suture, in that the Mesozoic foreland basin south of the suture contains lots of carbonate gravels of Late Paleozoic and Early Triassic ages in its proximal zone . The Devonian and Early Carboniferous strata preserved along the southern rim of the South Qinling are inferred to have formed in a rifting setting implied by synchronous alkaline magmatism. Continent-slope thin-bedded limestones, hemipelagites, and cherts comprise the Late Carboniferous¯Permian strata, and are closely associated with the Mianlue ophiolitic complex to the west ( Meng et al., 1996).
Alluvial and fluvial deposits comprise the Permo-Carboniferous successions of the North Qinling in response to the North and South Qinling collision in the Middle Paleozoic (Zhang, 1988). The Upper Triassic and Jurassic are missing in most parts of the Qinling orogen, but the Cretaceous and Cenozoic redbeds are common in various-scale fault-bounded basins. The Cenozoic sediments are up to 9 km in thickness in the Fenwei Graben immediately north of the Qinling ranges (Wang, 1987).
2.4 Ophiolitic complexes
Three zones of ophiolitic complex are present in the Qinling: the Mianlue zone between the South Qinling and the South China block, the Shangdan zone between the North and South Qinling, and the Erlangping zone within the North Qinling.
The Shangdan ophiolitic complex is bounded on the north by the Qinling Group and on the south by the Shangdan fault zone (Fig. 5). Original sequences have been dismembered into various-scale fragments that are now in fault contact with one another. This ophiolitic complex consists primarily of ultramafites, gabbros, basalts, diabasic dikes, pillow lava and radiolarian cherts (Fig. 5). Basalts are divided into two groups, one is calc-alkaline with enrichment in LREE, and the other is tholeiitic, showing depletion in LREE and flat distribution pattern. The LREE-enriched basalts are low in Ti, Nb, Ta, and show Th>Ta, Nb/La<0.8, Hf/Th>8, and Zr/Yb<3. CeN/YbN ratios range from 5 to 20. All these indicate an intra-oceanic island-arc setting . In comparison, LREE-depleted tholeiites show MORB characteristics, with ratios of Th/Ta and La/Ta close to 1, TiO2=1.65%, and Ti/V=22. The REE patterns are parallel to those of N-MORB, indicating that they were derived from depleted N-mantle . The Sm¯Nd mineral isochron of norite gabbros gives an age of 402.6±17.4 Ma (Li et al., 1989), consistent with radiolarian ages from the Ordovician to Silurian .
Fig. 5. Geologic map of Guojiagou area, showing main components of the Shangdan ophiolitic complex and structures. Refer to Fig. 2 for location of this map (Meng et al., 2000).
The Erlangping ophiolitic complex crops out within the North Qinling, in fault contact with the Qinling Group in the south and with the Kuanping Group in the north (Fig. 6). It is composed mostly of olivine gabbros, massive and pillow basalts, sheeted dikes and sills, radiolarian cherts and some marbles. Ultramafic rocks, such as peridotites, also exist, but are not well cropped out ( Liu et al., 1993). Massive basalts, mostly tholeiites, are the dominant component, showing slight enrichment in LREE and flat REE pattern. Their LaN/YbN and CeN/YbN ratios rang from 0.88¯2.91 and 0.88¯2.30, similar to those of N-MORB (Sun et al., 1996). Radiolarians in the cherts interlayered with the basalts indicate that the basalts are of Ordovician to Silurian age. The Erlangping ophiolite occurs to the north and shares the same age of the Danfeng arc massif. It is suggested that the Erlangping ophiolite represents relics of a back-arc basin formed in response to the northward subducting (present orientation) of Early Paleozoic Qinling ocean beneath the North Qinling.
Fig. 6. Geologic map of Erlangping area, showing main components of the Erlangping ophiolitic complex. Refer to Fig. 2 for location of this map (Meng et al., 2000).
The
Mianlue ophiolitic complex is present along the boundary between the South
Qinling and the South China block, and is particularly well exposed to the
west. It is correlated with ophiolites at the southern boundary of the Eastern
Kunlun because they are of similar ages and occur in identical tectonic
settings. Broken ophiolitic sequences in the western segment contain
ultramafites, gabbros, oceanic tholeiites, and radiolarian cherts ( Fig. 7). The ultramafites, most of which have been
altered into serpentinites, are inferred to be originally harzburgites and
dunites . They show LREE-depleted pattern with positive Eu anomaly, and both LaN/YbN
and CeN/YbN ratios range from 0.4 to 1.24 and 0.48 to
1.23. Gabbros show slight enrichment in REE, and are characterized by cumulate
and gabbro-diabasic textures. Diabasic dikes are all LREE-enriched . Basalts
are divided into two groups ( Lai et al., 1998). One is LREE depleted
tholeiites, with LaN/YbN ratios being 0.33¯1.01.
In addition, Eu is from 0.84 to 1.13 and no Eu anomaly has
been confirmed. All these data suggest that these basalts are MORB and derived
from depleted mantle . In contrast, the other basalt suite is LREE-enriched
tholeiites with LaN/YbN ratios being 1.84¯4.70 and CeN/YbN
ratios 1.82¯3.38. These basalts are also characterized by the
following facts: Th>Ta, Th/Ta=1.84¯4.70, Nb/La<0.6, and
Tb/Yb=0.10¯0.80, showing characteristics of island-arc volcanics
(Lai et al., 1998). Therefore, in addition to the ocean-crust lavas, some
island-arc volcanites are also involved into the Mianlue ophiolitic complex.
The age of the Mianlue ophiolitic complex has not been well constrained. It is
argued, however, that the ophiolite is of Carboniferous age according to
radiolarians in the cherts interlayered with the basalts , synchronous with the
ophiolites along the southern boundary of the Eastern Kunlun to the west .
Figure.7 Geologic map of Mianxian¯Lueyang area, showing components of the Mianlue ophiolitic complex and structures. Refer to for location of this map (Meng et al., 2000).
Eastern continuation of the Mianlue ophiolite is largely covered by south-directed Mesozoic thrusts, and thus has become a matter of debate. Recently, some ophiolitic fragments were found in the Shuizhou area (Fig. 2), supporting the Mianlue suture's eastward extension along the southern edge of the South Qinling. These ophiolitic relics include gabbros, basalts, diabasic dikes, and associated cherts, but few ultramafites are cropped out. The basalts are MORB in nature ( Lai et al., 1998b). The ophiolitic fragments in this area have not been dated, but are inferred to be Late Carboniferous to Early Triassic. The age inference is based on two facts: the Early Triassic deep-marine deposits are closely associated with the ophiolite, and the Devonian and Early Carboniferous sedimentation and magmatism are apparently related to the rifting that preceded the opening of the ocean, or the Paleo-Tethyan Qinling Ocean ( Meng et al., 1999). In addition, no ophiolitic fragment has been observed along boundary faults between subzones in the South Qinling. Late Paleozoic¯Early Triassic facies distribution shows an obvious southward deepening trend from continental through shallow-marine to deep-marine depositional environments, indicating that the Paleo-Tethyan Qinling Ocean should be located to the south of the South Qinling at that time. Thus the Mianlue suture should follow the southern edge of the South Qinling rather than enter it from the west.
