Permian magmatism and lithospheric deformation in ... - Science Direct

Russian Geology and Geophysics 49 (2008) 468–479 www.elsevier.com/locate/rgg

Permian magmatism and lithospheric deformation in the Altai caused by crustal and mantle thermal processes A.G. Vladimirov a, *, N.N. Kruk a, S.V. Khromykh a, O.P. Polyansky a, V.V. Chervov b, V.G. Vladimirov a, A.V. Travin a, G.A. Babin a, M.L. Kuibida a, V.D. Khomyakov a a

b

Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Received 26 December 2007

Abstract New structural and petrological data have been obtained for the zone of Siberia-Kazakhstan oblique collision for Permian time. In terms of classical tectonics, the area coincides with the Zaisan folded area produced by closure of the Char paleo-ocean in the Late Carboniferous. However, the extent, structure, and composition of magmatism at the Carboniferous-Permian (280±10 Ma) and Permian-Triassic (250±5 Ma) boundaries require an active control from Morgan-type lower mantle plumes (Tarim and Siberian plumes). Structure formation in the lithosphere and heat sources of magmatism have been simulated in a 3D model including lithospheric strain rates (with regard to viscosity layering) and subcontinental upper mantle convection. According to our model, heat supply from slab break-off and/or delamination of lithosphere is insufficient to maintain large-scale mantle-crustal magmatism in the case of oblique collision between 80–100 km thick plates (“soft collision”). The Late Paleozoic-Early Mesozoic Altai is considered as a model of a large hot shear zone, a particular structure produced by interference of plate- and plume-tectonic processes. Special attention is given to structural and petrological markers of plume tectonics (reported for the case of the Altai collisional shear system), with their diagnostic features useful for understanding geodynamics of other similar regions. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Geodynamics; plate tectonics; plume tectonics; mathematical modeling; lithospheric deformation; heat and mass transfer; petrological markers of plume activity; Altai collisional shear zone

Introduction The Permian-Triassic boundary was a key milestone in the Earth’s history being the time when Eurasia began to stabilize as a single lithospheric plate and when plumes (superplumes) began to control the tectonic setting. They were, especially, Siberian (North Asian) plume (Al’mukhamedov et al., 1999; Dobretsov, 1997, 2003; Dobretsov et al., 2005; Medvedev et al. 2006; Yarmolyuk et al., 2000; Yarmolyuk and Kovalenko, 2003) and the Tarim plume, for which ever growing evidence has been reported recently (Borisenko et al., 2006; Changyi et al. 2006; Mao et. al., 2005). There still remain two questions: (i) how to discriminate between the plumes associated with upper mantle convection or asthenospheric upwelling induced by plate interaction and those derived from lower-mantle sources and (ii) how to estimate the share of

* Corresponding author. E-mail address: [email protected] (A.G. Vladimirov)

plate- and plume-tectonic controls of regional structures, with the respective geological and metallogenic implications. To solve these problems, the best approach is the one by Kuznetsov (1964) who suggested to judge geological objects in terms of structure, composition, and age, three linked criteria of equal diagnostic value. Kuznetsov’s ideas were popular with Russian geologists in the 1960s and 1970s (Kuznetsov and Izokh, 1969; Kuznetsov and Yanshin, 1967; Letnikov, 1975) but became almost abandoned on the advent of innovative analytical techniques for determining compositions and ages of rocks to an error of less than 5% (Faure, 1986). The new techniques were based on isotope criteria (Rb-Sr, Sm-Nd, U-Th-Pb, Lu-Hf systems, and He ratios) diagnostic of mantle sources (Faure, 1986). These criteria are, however, often ambiguous, and the very basic concepts behind discriminating the mantle reservoirs have been doubted (Kostitsyn, 2004; Pushkarev and Nikitina, 2007). The rapidly increasing body of hotspot data from oceans free from effects of continental crust has brought more ambiguity rather than clarity (Courtillot et al., 2003; Foulger,

1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.06.006

A.G. Vladimirov et al. / Russian Geology and Geophysics 49 (2008) 468–479

2007; Puchkov, 2006). Thus the plate tectonic-plume tectonic interference acquires special importance as it concerns the heat sources that are responsible for the origin of continental orogens, such as the Ural suture or the Central Asian belt. It is reasonable to accept the idea that plumes may originate at different levels and may be, respectively, of at least three types (Puchkov, 2006): (i) primary (Morgan-type) plumes that rise from the lower mantle, (ii) intermediate plumes from the base of the transition zone, and (iii) Anderson-type upper mantle plumes that appear in response to plate motion. This study is an attempt to explore plate- and plume-tectonic controls in the case of the Altai collisional shear system. We use the “structure-composition-age” approach which allows reasonable boundary conditions for 3D models and their testing on a regional scale. The Altai system appears to be a good choice because its Permian-Triassic evolution was driven by two independent processes of oblique plate convergence and plume activity, and their interplay was a critical point of Eurasian history. We are trying to decide whether the mantle and mantle-crustal magmatism of the Altai was merely a response to the Kazakhstan-Siberia oblique collision or its explanation requires additional mantle sources of material and heat.

Tectonic framework and geodynamic history of the Altai collisional shear system The Altai builds the western flank of the Central Asian orogen (see Dobretsov et al., 1994; Mossakovsky et al., 1993; Sengör et al., 1993 for its tectonics and evolution). It includes the Ob’-Zaisan and western Altai-Sayan folded areas which in the Late Paleozoic-Early Mesozoic evolved as a single large tectonic unit of the Altai collisional shear system (Vladimirov et al., 2003). This study is confined to the Kazakhstan-Siberia junction commonly attributed to the Zaisan folded area and part of the Gobi-Zaisan Late Paleozoic orogen (as exposed in today’s erosion surfaces). The Gobi-Zaisan orogen is buried under Mesozoic-Cenozoic cover of the West Siberian plate in the north and extends into the Mongolian Gobi and China in the south (Fig. 1). The recent views of the Late Paleozoic Altai orogeny (Berzin and Kungurtsev, 1996; Buslov et al., 2003; Vladimirov et al., 2005) are that Kazakhstan and Siberia were converging gradually and rotating clockwise relative to each other during closure of the Char ocean; before the collision (in DevonianEarly Carboniferous), the Altai and Kazakhstan zones were active margins. The Altai-Mongolia microcontinent with Neoproterozoic crust (TNd(2-st) = 1.5–1.0 Ga) was sliding along the edge of Siberia, and the Char oceanic plate subducted obliquely beneath Kazakhstan and Siberia along two zones (Zharma-Saur and Rudny-Altai island arcs). After the ocean closure had completed by the Middle Carboniferous, the orogen evolved in an environment of left-lateral shear. The eastern (Siberian) sector of the Late Paleozoic Altai orogen includes the complexes of East Kazakhstan, and Rudny Altai and Gorny Altai terranes upon an Early Paleozoic

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basement, the Altai-Mongolia microcontinent, the KuznetskAlatau terrane which is a fragment of a seafloor rise with the TNd(2-st) = 0.85–0.65 Ga protolith age (Vladimirov et al., 1999), and the Kuznetsk sedimentary basin of a polygenetic origin (Polyansky et al., 2004). The western (Kazakhstan) sector has a simpler structure and consists mainly of volcanosedimentary and intrusive complexes that mark the island-arc phase in the Late Paleozoic orogenic history of Altai (Ermolov et al., 1977). The West Kalba (Char) zone is a key unit in the area. Its basement most likely lacks Precambrian complexes, and the exposed Devonian-Early Carboniferous volcanosedimentary rocks may be remnants of oceanic crust (Dobretsov et al., 1979, 1981; Ermolov, et al., 1983). The Char ophiolite suture mapped along the zone axis (Fig. 2) marks the presumable line of the Kazakhstan-Siberia junction and involves relics of oceanic high-temperature low-pressure metabasaltoids with the ∼400 Ma Ar-Ar muscovite age of metamorphism (Volkova et al., 2008). Suprasubduction processes on the active margin of Kazakhstan are recorded in island-arc magmatism and deposition in the Zharma-Saur zone (Saur volcanoplutonic rocks, C1t (Ermolov et al., 1977)). Reliable signature of coeval subduction-related processes on the Siberian active margin was found in Rudny Altai and in the Belouba basin (Berzin and Kungurtsev, 1996; Dobretsov et al., 2001; Lopatnikov et al., 1982; Shcherba et al., 1998; Shokalsky et al., 2000; Vladimirov et al., 2001). The particular setting of the Rudny Altai area, which in the Late Paleozoic simultaneously experienced subduction from the west (in the present frame of reference) and collision in the east (from the Altai-Mongolia microcontinent), apparently induced the initiation of the Irtysh shear zone. The age of the latter remains uncertain but must be no older than Late Devonian-Early Carboniferous according to geological evidence. The geometry of the colliding plates of Kazakhstan and Siberia, as well as the specificity of accretion and subduction, caused the asymmetric structure of the Altai system. The Altai orogen produced by the oblique collision bears signature of inheritance from the Late Devonian-Early Carboniferous Zharma-Saur, Rudny Altai, and Gorny Altai structures. This is evident especially from initiation and/or reactivation of the transcurrent faults of Zharma, West Kalba, Rudny Altai, and Gorny Altai, higher sedimentation rates in the Kuznetsk sedimentary basin, and spread of volcanoplutonic magmatism and batholith emplacement. Note that the main phases of strike-slip and extensional faulting strikingly correlate with acceleration of sedimentation in the Kuznetsk Basin (Vladimirov et al., 2003).

Kinematics of magma-guiding faults The Altai system underwent its main collisional phase in the Late Carboniferous during the closure of the Gobi-Zaisan (Char) ocean (Berzin and Kungurtsev, 1996; Buslov et al., 2003; Dobretsov et al., 1979, 1981; Ermolov et al., 1977, 1983; Polyansky et al., 2004; Vladimirov et al., 1999, 2005;

Fig. 1. Generalized tectonics of Zaisan folded area, a fragment of Altai collisional shear system. 1 — Early Paleozoic orogenic complexes of Kazakhstan and Siberia continents, undifferentiated; 2 — Late Paleozoic terranes of Zaisan folded area corresponding to active continental margins, undifferentiated; 3 — Char oceanic terrane; 4–6 — Late Paleozoic igneous complexes, divided according to ages into Late Triassic-Early Jurassic mafic dike belts (T3–J1) (4), Late Triassic-Early Jurassic batholiths (T3–J1) (5), and Middle Permian-Early Triassic subalkaline volcanics and rare-metal granite-leucogranite intrusions (P2–T1) (6); 7 — Kalba (Zharma)-type batholiths and bimodal volcanics and Kunush-type plagiogranite (P1); 8 — Argimbai- and Maksut-type subalkaline gabbro and picrite (P1); 9 — Devonian-Early Carboniferous batholiths along margins of Zaisan folded area, undifferentiated; 10 — Char ophiolite; 11 — transcurrent faults (numbers in circles along fault strike stand for fault names, keyd as: 1 — Arkalyk, 2 — Zharma, 3 — Zhana-Bugaz, 4 — Boko-Baiguzin, 5 — Chara, 6 — West Kalba, 7 — Kalba-Narym, 8 — Irtysh, 9 — Kedrovo-Butachikha, 10 — Loktevo-Zyryanka, 11 — Beloretsk-Markakul, 12 — Loktevo-Karairtysh); 12 — major shear zones of viscoplastic and brittle-plastic flow, with their names abbreviated as ZhSZ — Zharma, ChSZ — Char, ISZ — Irtysh; 13 — geometry of motion.

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Fig. 2. Generalized structure of Char shear zone, modified after (Buslov et al., 2003; Dobretsov et al., 1979, 1981). 1 — serpentinite melange; 2 — allochthone stratigraphic units (O2-C1); 3 — faults (a — observed, b — inferred); 4 — central part of transpression zone marked by allochthones and serpentinite melange. Arrows show geometry of major and subsidiary strike-slip faults.

Volkova et al., 2008). That period of activity was associated with initiation and repeated reactivation of strike-slip faults and/or complex systems of oblique, reverse, and thrust faults in the Chingiz-Tarbagatay-Zharma and Irtysh shear zones and in the Char ophiolite suture (Fig. 2). The rugged topography and numerous exposures have drawn to the surface features of viscoplastic and brittle-plastic deformation within each zone. Our objective was to determine the style of deformation, as well as kinematics and ages of faults, which are the notably correlated characteristics in the Gobi-Zaisan orogen as a whole (An Yin, pers. commun.). The Irtysh system is a major regional zone of deformation (Chikov and Zinoviev, 1996; Dobretsov et al., 1994; Khoreva, 1963; Laurent-Charvet et al., 2003; Mossakovsky et al., 1993; Sengör et al., 1993; Vladimirov et al., 2003) including the Irtysh shear zone proper and the satellite Northeastern shear zone (Fig. 1). In earlier paleo-tectonic reconstructions, the complexes of the Irtysh shear zone were commonly interpreted as having experienced two major deformation events: an earlier event of right-lateral shear about 270 Ma (Sengör et al., 1993) and later left-lateral shear at 255–250 Ma. The ages and geometry of motion were updated later (Travin et al., 2001) due to structural and petrological studies and Ar-Ar isotope dating of syntectonic micas and amphibole from blastomylonite after schists, amphibolite, and granite. Most samples from sites along the Irtysh shear zone showed nearly identical Ar-Ar

ages (within analytical error) of 281±2.4, 277±7, 283±7, 283±1.6, and 282±1.5 Ma, with only two younger dates of 269±2.3 and 264±3 Ma on muscovite and feldspar, respectively. The new Ar-Ar ages record two large events of leftlateral viscoplastic and brittle-plastic deformation in the Irtysh shear zone (285–270 and 270–260 Ma, respectively). It is worthy of note that independent U-Pb dates for the Kalba batholith belt on the southwestern flank of the Irtysh shear fall in the same interval between 290 and 274 Ma (Vladimirov et al., 2001). Syntectonic rare-metal pegmatite of the Asu-Bulak deposit have Ar-Ar muscovite and lepidolite ages of 295±4 and 292±4 Ma, respectively. The syndeformation origin of the multistage Kalba batholith belt have been proved unambiguously by structural measurements in the area of the Bukhtarma water reservoir where the Early Kalba granite intrusions follow zones of weakness. Injections of granitic material in these zones have different thicknesses down to thin veins. The veins, along with the metamorphosed country rock, are deformed into shear bands or fill extensile features while preserving the intrusive contacts. The fact that syntectonic granites are sheared and mylonitized is evidence that shearing continued long after the first batches of granitic magma had emplaced (Chikov and Zinoviev, 1996; Vladimirov et al., 2005). The two events of left-lateral motions in the Irtysh shear zone are consistent with structural and petrological data, as

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well as with multisystematic U-Pb, Rb-Sr, and Ar-Ar dating of high-grade tectonic slices which are mapped as competent structures all along the zone. They are of two main types being composed of epidote-amphibolite facies kyanite-staurolite rocks (Predgornensky et al.) or high-temperature low-pressure (HT/LP) rocks, including those of the Altai-Scotland-type metamorphic core complexes (Checheksky et al.). The discussion on the age and genesis of the high-grade slices imbedded in a greenschist matrix of the Irtysh shear zone has a fifty-year long history. They have been interpreted either as Paleozoic metamorphic complexes produced by thermal effects from gabbro and granitic intrusions (Chikov and Zinoviev, 1996; Khoreva, 1963; Vladimirov et al., 2005) or as exhumed fragments of the Precambrian basement (Bespaev et al., 1997; Ermolov and Polyansky, 1980). Recent structural, petrological, and geochronological data support the Late Paleozoic age of metamorphism (Travin et al., 2001): Ar-Ar ages of 280±2.1 and 271±8.0 Ma, respectively, on muscovite and biotite for staurolite schists of the Predgorny block and a stable plateau of 282±2.3 Ma in the Ar-Ar age spectrum of hornblende from clinopyroxene amphibolite of the Sogren block. Note that these dates coincide with the Ar-Ar ages of micas from tectonized greenschists of the matrix. Migmatite leucosome in rare layers of garnet-biotite-sillimanite-feldspar metapelite gneisses from the Sogren block composed mainly of migmatized ultramafic rocks (HT amphibolite facies) has a U-Pb zircon age of 264±2 Ma, and migmatite leucosome from the Chechek granite gneiss dome has a similar U-Pb zircon age (260±8 Ma); the dome is a model Altai-Scotland-type metamorphic core complex in the Irtysh shear zone. As we mentioned, the Char ophiolite suture separates Early Paleozoic orogenic structures of Kazakhstan and Siberia (Figs. 1 and 2). The suture combines features of a thrust-fold belt and a deep fault (Belyaev, 1985; Bulin et al., 1969; Patalakha and Belyi, 1980; Polyansky et al., 1979; Rotarash and Gredyushko, 1974) traceable in geophysical fields for depths of more than 100 km. The fault shows up in erosion surfaces (Fig. 2) as serpentinite melange mostly along the northeastern side where the slip planes have nearly vertical dips. According to Rotarash and Gredyushko (1974), serpentinites were expulsed during later tectonism. Patalakha and Belyi (1980) likewise suggested that the ultramafic rocks in the Char ophiolite suture are protrusions exhumed by shear. Note that serpentinite melange is localized along both deep faults and bottoms of low-angle trusts (Belyaev, 1985). To put it different, the Char suture is a system of left-lateral strike-slip faults and widespread duplex (palm-tree) structures. This inference agrees with independent results by Buslov et al. (2003). The Zharma shear system lying between island-arc complexes associated with subduction beneath Kazakhstan is the least explored. There is published evidence (Polyansky and Tyan, 1978; Ponomareva et al., 1985) that erosion along the zone exposes tectonic slices dissimilar in composition and type of metamorphism (HP/LT and HT/LP). The wall rocks in both cases bear signature of left-lateral viscoplastic and brittle-plastic flow.

Quasi-3D simulation of shear deformation in collisional settings The above data on phases, scale, and kinematics of deformation, metamorphism and magmatism indicate that motion on all transcurrent faults during closure of the Gobi-Zaisan (Char) paleoocean (Late Carboniferous) was left-lateral strike slip at clockwise rotation of Kazakhstan and Siberia; all transcurrent faults, including the Char ophiolite suture, originated long before the collision and have preserved their significance till the present. Therefore, the pattern of large faults and their subsidiary (pinnate, cross, etc.) faults, with fault intersections, have undergone almost no change for the past 200–300 Ma being sealed in the lithosphere of a single continent of Central Asia. Local deformation did exist but relative motion of blocks never exceeded tens or few hundreds of kilometers. Quasi-3D finite-element simulation (Bird, 1999; Polyansky, 1998, 2002, 2006) turned out to be an appropriate tool to explore shear deformation in thermally and rheologically inhomogeneous continental lithosphere. Models of this kind can simulate active rifting with incorporated gravity instability at the core-mantle boundary and passive rifting with userspecified instantaneous strain rates along plate boundaries (Polyansky, 2006). For the Altai collisional shear system, we specified instantaneous strain rates both in the crust and lithospheric mantle along the boundaries of Kazakhstan and Siberia, with reference to their real geometry, presumable lithospheric thicknesses, and the fault pattern in the present framework. The finite-element grid was made in a way to account for the position of major transcurrent faults, from the Main Sayan Fault parallel to the western edge of the Siberian craton to the fault border of the Jonggar basin in the south of the Altai system. The grid consisted of 6300 nodes and over 3000 triangular elements, including about 300 curved elements of faults (Fig. 3, a). In the first test we assumed a homogeneous lithosphere, 100 km of total thickness (46 km thick crust plus 54 km thick lithospheric mantle), a heat flux of 20 or 50 mW/m2 in different versions, and a flat surface topography with a uniform elevation of +1000 m. The fault geometry was specified proceeding from the today’s lithospheric structure. The faults were assumed to be vertical with slip in any direction as a function of local stress, at a friction coefficient of 0.17. The other parameters were as in (Polyansky, 2006). The boundary conditions implied constant strain rates on the opposite northeastern and southwestern sides of the model domain and mixed conditions along the lateral sides (Fig. 3, b). The main objective was to explore possible mechanisms of the oblique plate convergence between Kazakhstan and Siberia and the formation of the Altai collisional shear system. We obtained three deformation patterns for different conditions along the lateral boundaries: an oblique collision with right or left shear and a frontal collision. In the case of oblique collision, motion on the lateral boundaries was allowed only along the Altai system and could be dextral or sinistral


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Fig. 3. Sketch of plane stress in Kazakhstan-Siberia oblique collision. a — finite-element grid (1), fault elements (2), geological boundaries (3); b — boundary conditions: vectors show motion of Kazakhstan boundary (two versions), dashed line is fixed boundary of Siberia, meeting triangles with arrows mark absence of motion along lateral boundaries at free motion in orthogonal direction.

depending on the direction of motion of the Kazakhstan plate. Figure 4, a shows computed horizontal strain rates (directions and amount in mm/yr, color scale) for a transpression model. However, vertical strain rates turned out to be more informative. Zones of positive and negative strain rates can be interpreted, respectively, either as mountains (lithospheric thickening) or basins (lithospheric thinning). The model domain in the left-shear version split into two large zones of positive (southwest) and negative (northeast) strains (Fig. 4, b), with the mountains of eastern Kazakhstan and Rudny, Gorny, and Mongolian Altay in the positive zone and the Altai foothills, the Kuznetsk Basin, the North and South Minusa basins, and the Lake Valley (Western Mongolia) in the negative zone. The simulation results are basically consistent with the real framework. The model intermontane basins (e.g., the Zaisan, Kurai, and Chuya basins in Altai) are most often bordered by or tangent to transcurrent faults, and their planiform geometry corresponds to pull-apart basins. Of special interest is the Gobi-Zaisan-Tien Shan structure which has no prominent manifestation in the shear models. The results would appear reasonable if we interpreted the axis of the Altai system (Char zone) as the place of maximum uplift in transpressional mountain building associated with the Kazakhstan-Siberia oblique collision, but this interpretation contradicts the absence of classical orogenic basins in the area. Sengor et al. (1993) did have some reasons to treat the Irtysh shear zone as a major regional strike-slip fault and to doubt the collisional origin of the Altai as a whole. In order to clear up the point, we simulated upper mantle convection for the case of oblique convergence between Kazakhstan and Siberia.

3D simulation of heat and mass transfer in convecting upper mantle at oblique continental collision The evidence of dynamics and kinematics of modern major transcurrent faults in the Altai collisional shear system, along with our modeling results, provide an idea of geodynamic processes associated with closure of the Paleoasian ocean (Fig. 5). Collisions of continents (or microcontinents) commonly lead to slab detachment in subduction zones and opening of asthenospheric windows, as it is known from seismic tomography, 2D modeling, and geological data from the Mediterranean and Pamirs-Himalayan areas (Davies and Blanckenburg, 1995; Khain et al., 1996). We performed 3D simulation of the temperature pattern in the upper mantle beneath an Altai-type collisional orogen to assess the role of ascending mantle flow related to slab break-off as a source of mantle and crustal magmatism. The model included the basic parameters and formalism as in (Chervov, 2002; Tackley, 1996; Tychkov et al., 2005a, b; Yakovlev et al., 2007). The lithosphere was simulated as a 120 km thick conductive layer attenuated to 70 km within a 100 km wide collisional corridor. The configuration of the latter was specified taking into account the peculiarity of the Kazakhstan-Siberia collision (Vladimirov et al., 2003, 2005) in which the interaction (and ensuing deformation) involved complexes on the ocean and continent margins rather than the thick Precambrian cores of the two continents. Our modeling results (Fig. 6) differ from those for the area of India-Eurasia collision (Khain et al., 1996) where interaction of thick Precambrian cratons caused lithospheric thickening to 200 km in the collision zone. Note in particular that mantle flow in the case of Pamirs-Himalayas (India-Eurasia

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Fig. 4. Model of collisional shear deformation of lithosphere. a — horizontal strain rates, color scale is at 1 mm/yr interval; b — vertical strain rates, with zones of thick (green) and thin (blue) lithosphere. Boundary conditions correspond to Kazakhstan moving eastward relative to fixed Siberia.

collision) is Anderson-type upwelling, most likely in response to plate motion. An ascending flow of this kind is obviously able to generate mantle magmatism and supply heat for crustal anatexis. Unlike the India-Eurasia collision, there is no upwelling (Anderson-type plume) in the case of the Altai system where we presume an asthenospheric window corresponding to the zone of oblique collision. On the contrary, there is convective

downwelling, with the size and geometry of flow varying along the zone strike: it either covers the entire region of thin lithosphere or shifts toward one edge of the asthenospheric trap. Note that ascending flow in convective cells is always along the margins of thick-lithosphere zones which in our model correspond to the edges of the converging continents and are 100 km or more off the collision axis. These are exactly the borders of the Altai system where the batholith


Fig. 5. Formation of Altai collisional shear system, with regard to asymmetry of orogenic structures and detachment of two slabs subducted beneath Kazakhstan and Siberia. See an orogen with an asthenospheric window — thin lithosphere (dark gray) (Vladimirov et al., 2003).

belts are localized (Figs. 1, 5). However, although the structural control of batholith emplacement is obvious, the large-scale remelting of crust would require much more heat. According to the modeling data, an Anderson-type plume associated with slab break-off can maintain only a very brief pulse of mantle magmatism (no longer than 5 Ma) at the beginning of collision. Then, interaction can continue only as “cold shear” (in an oblique collision) while the heat supply required for magmatism can come from radiogenic or crustal dissipation sources, which appears very unlikely in our case.

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Therefore, plate motion in the Altai system could not provide enough heat to maintain the long and diverse mantle and crustal magmatism, large-scale deformation, and hightemperature metamorphism of rocks. A reasonable explanation of the controversy is that the Permian-Triassic geodynamic activity of the Altai collisional shear system was driven by independent heat sources other than plate motion. For this, Morgan-type plumes and superplumes (Courtillot et al., 2003; Puchkov, 2006), namely, the Tarim and Siberian plumes, appear to be the only candidates. The geodynamic control from the Siberian superplume which acted since the PermianTriassic boundary has been largely discussed (Al’mukhamedov et al., 1999; Dobretsov, 1997, 2003; Dobretsov et al., 2005; Medvedev et al. 2006; Yarmolyuk et al., 2000; Yarmolyuk and Kovalenko, 2003) and needs no comment. In this study it is pertinent rather to focus mainly on Permian igneous complexes related to the Tarim plume. Preliminary data on compositions and ages of Permian igneous rocks were published in (Dobretsov et al., 1981, 2001; Ermolov et al., 1977, 1983; Lopatnikov et al., 1982; Shcherba et al., 1998; Shokalsky et al., 2000; Vladimirov et al., 1997, 2001), and the discussion below concerns with the most recent evidence.

Petrological signature of the Tarim plume The Permian was the time of large-scale magmatism in the Late Paleozoic orogenic area of East Kazakhstan which involved all structural zones. The magmatic activity produced the complexes of subalkaline gabbro (Argimbai, 293±2 Ma) and picrite dolerite (Maksut, 280±4 Ma) in the Char zone and gabbro and plagiogranitoids (Kunush, 307–299 Ma) in the

Fig. 6. 3D model of subcontinental upper mantle convection with a zone of thin (120 km) lithosphere between two thick plates (200 km). Patterns in sections are temperatures, with contour lines at 100 °C. See descending mantle flow in plane (xz) beneath zone of thin lithosphere (vortex? flow).

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Fig. 7. Generalized geology of Argimbai intrusive belt. 1 — basaltic andesite of Tersairyk Formation (C1t2- v); 2 — calcareous-carbonaceous deposits of Arkalyk Formation (C1v2); 3 — orogenic rather sedimentary deposits of Bukon Formation (C2); 4 — orogenic volcanosedimentary deposits (C2–3); 5 — gabbro-diabase and diabase of Argimbai complex; 6 — picritic porphyry and picritic diabase of Maksut complex; 7 — faults; 8 — Quaternary deposits; 9 — major intrusions (numbers in circles stand for their names, keyed as: 1 — Argimbai, 2 — Podkhozhye group, 3 — Shokzhal group, 4 — Zosimov, 5 — Pridorozhny, 6 — Karasu, 7 — Petropavlovsk group, 8 — Komsomolsky group, 9 — Kokpekty).

Kalba-Narym zone. It was also the time when batholith emplacement began in the Kalba-Narym and Zharma-Saur zones (295–274 Ma) and subalkaline volcanoplutonic associations formed in all three zones. The volcanoplutonic complexes exist as central-type edifices (Sirektas, Kokon, Tastau, Aktobe, Kalguta, etc.) eroded to different degrees (Ermolov et al., 1977, 1983; Shcherba et al., 1998). The Argimbai gabbro belt extends for more than 100 km along the Char zone (Fig. 7). It is a chain of sills and dikes composed of two complexes of Argimbai gabbro and plagiosyenite and the younger Maksut picritic rocks. The Argimbai complex has a wider spread and consists of gabbro, essexite gabbro, and plagiosyenite. The Argimbai gabbroic rocks show a high alkali enrichment (4–5 wt.% Na2O and 1.5–2 wt.% K2O) and high TiO2 (1.5–2 wt.%), P2O5 (0.5–0.7 wt.%), REE (a total of 190–270 ppm), LREE ((La/Yb)N = 9–10), Ba (780–1000 ppm), Sr (580–980 ppm), Zr (240–380 ppm), and Rb (11–36 ppm), i.e., they were apparently derived from an enriched mantle source. The younger Maksut rocks are picritic dolerite and olivine gabbronorite. They inherit the major- and trace-element compositions of the Argimbai association: belong to the medium-alkali series (2–4 wt.% Na2O and 0.7–1.2 wt.% K2O) and keep high REE and trace-element abundances (70 ppm REE, up to 240 ppm Ba, 830 ppm Sr, 8 ppm Rb, and 90 ppm Zr). The mafic magmas of the Argimbai belt formed in a succession of Mg# regularly increasing and SiO2 and alkalinity decreasing, correspondingly, from early (Argimbai gabbro-plagiosyenite complex) to late (Maksut picritic complex) pulses, with progressive involvement of low-melting components (growing melting degree). Gabbro-plagiosyenite rocks have a U-Pb SHRIMP-II zircon age of 293±2 Ma, and the picritic rocks have ages of 278±3 and 280±3 Ma determined by the 40Ar/39Ar method on hornblende and phlogopite, respectively (Khromykh et al., 2007). The Kunush plagiogranite belt of small subvolcanic intrusions and dikes in the Kalba-Narym zone is more than 200 km long (Fig. 1). It consists of biotite plagiogranite or plagiogranite porphyry which show a generally low-K calc-

alkaline chemistry but high Al2O3, Sr, and Eu and low Y and HREE as in peraluminous rocks (Arth, 1983). Highly aluminous plagiogranites commonly result from high-pressure (10– 12 kbar) dehydration melting of a mafic source (Rapp and Watson, 1995). This origin is consistent with Sm-Nd data: the Sm-Nd model age of the protolith in the Kunush plagiogranite is 0.52 Ga (εNd(T) = +6.7) while that of metabasalt in the Char ophiolite belt is 0.46 Ga, εNd(T) = +6.8 (Safonova, 2005; Volkova et al., 2008). According to the trace-element and Nd isotope compositions, the Kunush plagiogranite may have been derived from molten base of the Kalba-Narym oceanic turbidite basin. The U-Pb zircon SHRIMP-II age of the plagiogranite association is 307–299 Ma (Kruk et al., 2007). The Kalba-Narym and Zharma batholith belts occupy an area of over 15,000 km2 (Fig. 1). We report the compositions and ages of rocks in the belts using an example of the Kalba-Narym zone which includes the Early Kalba granodiorite-granite, Late Kalba granite-leucogranite, and Monastyr leucogranite complexes (Lopatnikov et al., 1982; Ponomareva and Turovinin, 1993) composed of peralkaline potassic granodiorite, granite, and leucogranite. The Kalba granitoids share high abundances (above the crustal average) of rare alkalis, Be, Sn, and Nb. Note that the Early Kalba granitoids host several deposits and occurrences of Li-Ta-Nb rare-metal pegmatite and hydrothermal veins with Sn-W mineralization while the late complexes are almost ore-barren. U-Pb data show quite a broad range of ages from 295–274 Ma for the Early Kalba complex, to 253–245 Ma for the Late Kalba complex, and as young as 231—225 Ma for the Monastyr complex (Vladimirov et al., 2001). The Ar-Ar ages of raremetal mineralization are 294±4 and 292±4 Ma on muscovite from greisens and on lepidolite from Li pegmatite, respectively. The Nd model ages of the Kalba granitoids are εNd(T) = 0...+3 and TNd(2-st) = 0.77–1.0 Ga. Thus, they have intermediate Sm-Nd characteristics between the mafic base of the Kalba-Narym terrane and its sedimentary cover (εNd(T) = −0.9...−2.2; TNd(2-st) = 1.18–1.34 Ga). The paleovolcanic edifices are found throughout the Zaisan folded area (Sirektas and Kokon in the Zharma-Saur zone,


Tastau in the Char zone, Aktobe and Kalguta in the KalbaNarym zone, and Serzhikha in the Rudny-Altai zone). They underwent erosion to different degrees which exposed both extrusive and subvolcanic facies. The rocks are mainly of acidic compositions (dacite, granodiorite, granite, or rhyolite) but there are also stocks of basaltic andesite (Aktobe), subvolcanic gabbroics, monzonitoids, and syenitoids (Tastau and Sirektas), and mafic dikes (in almost all structures). The major- and trace-element compositions and PT data on melt inclusions available for the Aktobe edifice (Titov et al., 2001) indicate that silicic magma generated in the lower crust at high pressure (P ∼ 10−12 kbar) and high temperature (900– 1200 °C). This magmatism can be accounted for by injection of HT mantle melts into subcontinental crustal base and “advanced” anatexis of high melting degrees (20% for metamorphosed mafic rocks and 40–50% for metapelitic rocks) in local sources in the lower crust. The coexistence, in comparable amounts, of rocks corresponding to dissimilar melting degrees (dacite and rhyolite), along with the small volume of magmatism, is evidence of anatexis in a high-gradient temperature field, i.e., a very hot (at least 1400 °C) heat source located right at the base of continental crust. The U-Pb SHRIMP-II zircon age inferred for the Sirektas leucogranite is 289±7 Ma (Kuibida et al., 2004). Thus, the available data indicate (i) an enriched source of mafic magmatism and an important contribution of mantle magma to formation of granitoids and bimodal volcanoplutonic associations, and (ii) compositionally diverse magmatism within a short span of 300–280 Ma (Late Carboniferous-Early Permian) coeval with the activity of the Tarim plume (300– 270 Ma) in northwestern China (Mao et al., 2005; Borisenko et al., 2006).

Conclusions The Altai area in the Late Paleozoic-Early Mesozoic was a model of a large hot shear system, a particular structure produced by interference of plate- and plume-tectonic processes. Accretion and collision obviously provided structural control while the Tarim and Siberian plumes acted as heat sources able to maintain the long duration and diversity of mantle and crustal magmatism, intense deformation, and high-grade metamorphism of continental crust. A special discussion on the role of hot shear systems in the tectonic evolution of Altaids, with implications for continental crustal growth, requires more data and new ideas, often alternative to the common concepts (Puchkov, 2006; Sengor, 2006), and is beyond the scope of this study. We dedicate this paper to the late Sergei A. Tychkov. The study was supported by grants 07-05-00853 and 07-05-00980 from the Russian Foundation for Basic Research. It was carried out as part of SB RAS Integration Projects 116, ESD-7.10.2, Project RNP-2.1.1 of the Ministry of Science and Education, and Program 702 “Development of Scientific Potential of Higher Education, 2006–2008”.

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