Two contrasting petrotectonic domains in the Kokchetav megamelange (north Kazakhstan): Difference in exhumation mechanisms of ultrahigh-pressure crustal rocks, or a result of subsequent deformation? (original) (raw)
Related papers
The Island Arc, 2000
High-to ultrahigh-pressure metamorphic (HP-UHPM) rocks crop out over 150 km along an east-west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet-like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north-dipping, fault-bounded orogen-parallel units (I-IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high-pressure amphibolite through quartz-eclogite and coesite-eclogite to diamond-eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780-1000°C, P = 37-60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730-750°C, P = 11-14 kbar; Unit IV: T = 530°C, P = 7.5-9 kbar) and bottom (Unit I: T = 570-680°C; P = 7-13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top-to-the-north) and top (top-to-the-south) of the pile. The orogenscale architecture of the massif is sandwich-like, with the HP-UHPM nappe group juxtaposed across large-scale subhorizontal faults, against underlying low P-T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP-UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower-grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30-40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab-breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP-UHPM rocks.
American Mineralogist, 2016
The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950-1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800-900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900-950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528-521 Ma), peak (528-522 Ma), and peak to retrograde metamorphism (503-532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20-25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models.
Island Arc, 2001
The central part of the Kokchetav Massif is exposed in the Chaglinka-Kulet area, northern Kazakhstan. The ultrahigh-pressure-high-pressure (UHP-HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I-IV) metamorphosed under different pressure-temperature (P-T) conditions. The coesite-and diamondbearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneissdominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP-HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW-SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by largescale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest northwestward extrusion of UHP Unit II.
The Island Arc, 2000
To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh-pressure and high-pressure (UHP-HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat-Kol regions, northern Kazakhstan, were examined. The UHP-HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet-amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat-Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO 2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat-Kol regions indicate that the metamorphic grades correspond to epidote-amphibolite, through high-pressure amphibolite and quartz-eclogite, to coesite-eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP-HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP-HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low-pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP-HP unit during the exhumation process, and that all of the UHP-HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.
Island Arc, 2001
The Kokchetav Massif of Kazakhstan includes high to ultrahigh-pressure (HP-UHP) metamorphic rocks (some of which were recrystallized at depths in excess of 150 km), juxtaposed against much lower pressure metamorphic components. We investigated the relationship between the HP-UHP metamorphic unit and the low pressure (LP) unit (Daulet Suite) in the Sulu-Tjube area, where the metamorphic rocks have previously been interpreted as constituting a megamelange with subvertical structural attitudes. Analyses of fold structures suggest that the HP-UHP metamorphic unit overlies the LP unit across a west-dipping subhorizontal boundary. In addition, kinematic indicators display top-to-the-north senses of shear along the tectonic contact between the two units, indicating that the HP-UHP unit has been extruded northward onto the LP unit. Following the juxtaposition of the two units, upright folds developed in both units, and these are associated with the previously reported steeply dipping metamorphic foliations. These data have important implications for the mode of exhumation of the UHP rocks from upper mantle to shallow crustal depths.
Geology of the Kokchetav UHP-HP metamorphic belt, Northern Kazakhstan
The Island Arc, 2000
Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure-high-pressure (UHP-HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP-HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.
Doklady Earth Sciences, 2010
Ultra high pressure (UHP) and high pressure (HP) metamorphic complexes in continental subduc tion zones are spatially associated with younger medium pressure (MP) and low pressure (LP) meta morphic rocks . It is a controversial question whether these MP-LP rocks are formed by the con tact effect of exhumed hot nappe stack of UHP-HP rocks on the underlying rocks (by analogy with meta morphic base of obducted ophiolite allochthon) or result from later syncollisional metamorphism related to crustal thickening and granite magmatism. MP and LP metamorphic complexes with inverted metamor phic zoning are known in the collisional Himalaya nappe thrust system . Structurally, they are con fined to the overthrusting of the Himalayas onto the Indian continent along the Main Central Thrust. The MP (garnet-kyanite-staurolite) and LP (sillimanitebiotite and biotite) rocks were formed at the initial stage of thrusting (25-15 Ma) and exhumed to the surface by later thrusts and denudation .
The Aktyuz Metamorphic Complex in the western Tianshan Mountains of Kyrgyzstan comprises high-pressure (HP) granitic gneisses and metasediments hosting mafic HP lithologies including eclogite. Geological relationships and geochemical-isotopic data of the different rock types of the Aktyuz Complex have been interpreted as a continental crustal fragment and the mafic rocks as former dykes. This study combines Sm–Nd and 40 Ar/ 39 Ar mineral ages, and P–T pseudosection modelling for a single retrogressed eclogite sample in order to shed light on the P–T–t evolution of the mafic HP rocks. The eclogite experienced a clockwise prograde P–T path with peak metamorphic conditions of ca. 2.1 GPa at ca. 670 °C, corresponding to a burial depth of ca. 70 km. The post-peak metamorphic P–T conditions suggest isothermal decompression and moderate initial uplift of the eclogite to ca. 40 km depth. A Sm–Nd isochron age of 462 ± 7 Ma for garnet, omphacite, and whole-rock is interpreted as time of cooling and retrogression of the eclogite sample below 650 to 600 °C at pressures b 1 GPa. We suggest that the HP rocks of the Aktyuz Complex represent deeply subducted continental crust of one of the Palaeo-Kazakhstan terranes. During exhumation of the metasediments (now paragneiss), the mafic rocks were juxtaposed at variable depth and at low temperature (b 400 °C, and in the case of the reported sample after ca. 462 Ma) with the more buoyant paragneisses in the subduction channel. This channel flow exhumation model can account for the different high-temperature petrological and geochronological histories of paragneiss and eclogite.
Journal of Metamorphic Geology
The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high-P (HP) mafic blocks and ultra-HP Grt-Cld-Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt-Cld-Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back-arc basin or arc-related volcaniclastics. Peak metamorphic conditions of the Grt-Cld-Tlc schist reached~580°C and 2.85 GPa corresponding to a maximum burial depth of~95 km. A Sm-Nd garnet age of 475 AE 4 Ma is interpreted as an average growth age of garnet during prograde-to-peak metamorphism; the low initial eΝd value of À11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic m elange. Furthermore, channel flow cannot only explain a tectonic m elange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural 'dome' with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.
Doklady Earth Sciences, 2010
Marginal parts of cratons preserve valuable infor mation on features of the evolution of the lithosphere that are controlled by different geodynamic events. This explains the particular interest in the problems of reconstruction and evolution of accretion and colli sion structures of the continents at the craton bound aries . The Yenisei Ridge is an accretion-collision orogen on the western margin of the Siberian Craton and it is important for understanding the tectonic evo lution of mobile belts surrounding ancient cratons and for the solution of the problem discussed in the mod ern literature on the entrance of the Siberian Craton in the composition of ancient supercontinents, Pangea and Rodinia.