Exhumation Of Ultrahigh-pressure Rocks Research Articles

U-Pb zircon ages for a collision-related K-rich complex at Shidao in the Sulu ultrahigh pressure terrane, China.

The Shidao complex at the eastern extremity of the Sulu ultrahigh pressure (UHP) terrane is composed of oldest pyroxene syenite, quartz syenite and youngest granite intrusives of the K-rich shoshonitic series. U-Pb zircon dating yields nearly concordant ages of 225 ± 2, 211 ± 3 and 205 ± 5 Ma for pyroxene syenite, quartz syenite and granite, respectively. The ages closely postdate the 240 to 220 Ma UHP metamorphism and correspond to the rapid cooling and exhumation of UHP rocks. A close genetic relation may exist between the formation of the Shidao intrusives and the continental collision and UHP metamorphism. However, the K-rich Shidao intrusive rocks are different from common syn-collisional granites in association of K-rich and granitic magmatism. A breakoff model is postulated to explain the formation of the complex. The breakoff of the subducting slab caused the rapid exhumation of the UHP terrane. Mantle upwelling resulted in basaltic magmatism and formation of the K-rich complex. Formation of the Shidao complex marked the cessation of the UHP metamorphism and the oldest ages of 225 ± 2 Ma of the complex is the minimum timing for the UHP metamorphism.

U. Ring, M.T. Brandon, G. Lister and D.S. Willet (Eds.) Exhumation Processes: Normal Faulting, ductile Flow and Erosion. (Geological Society: Special Publication 154), 1999, viii + 378 pp. List Price: £89.00 GSL price: £39.00. ISBN 1-86239-032-0.

This Special Publication of the Geological Society followed an International Conference with the same theme. The main aim of the volume according to the editors was to sample ‘frontier research’ on the processes that lead to the exhumation of metamorphic rocks in ancient and modern orogens. The volume contains sixteen papers arranged into three sections. In understanding of exhumation processes, in common with other growth interest areas, the Earth Sciences has generated a raft of often poorly defined terminology. The initial paper by the editors provides, therefore, a useful review of the exhumation problem defining the terms of reference, identifying the tectonic processes, the tectonic settings and criteria that might be diagnostic for specific exhumation processes. The emphasis on the origins and exhumation of ultra-high pressure rocks is clearly of special interest to the editors but their deliberations do distort an otherwise well balanced review. The examples used to illustrate each section are, nevertheless, well chosen and the paper provides an excellent introduction for any student wishing to embark on a study of exhumation processes. The arrangement of the remaining papers into the categories of ‘Subduction-related accretionary wedges’, ‘Collisional belts and intra-continental convergence’ and ‘Lithospheric extension’ seems a little forced and not all are appropriately placed. While the groupings are indicated in the contents page they are not separated in the body of the volume. The first two papers examining exhumation in Subduction-related settings are concerned with Franciscan type HP/LT rocks. Sedlock presents field and structural data to evaluate the mechanisms responsible for the exhumation of the blueschists in western Baja California. Erosion, strike-slip faulting, buoyancy and ductile flow are considered to have played only minor roles while normal faulting is confirmed as the principal exhumation mechanism. In …

Thermochronologic constraints on deformation and cooling history of high‐ and ultrahigh‐pressure rocks in the Qinling‐Dabie orogen, eastern China

The Hong'an block is the best place to study the exhumation of high‐pressure (HP) and ultrahigh‐pressure (UHP) metamorphic rocks in the Qinling‐Dabie orogen of eastern China because it lacks the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan. We measured timing of deformation and rate of cooling of the HP‐UHP rocks by 40Ar/39Ar analyses of synkinematic minerals from tectonites of key structural zones in the Hong'an and Tongbai Shan. Normal‐sense shear along the north dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237 and 231 Ma; this detachment facilitated the bulk of the exhumation of the HP‐UHP rocks. Our new 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages of 245–240 Ma, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5–25 mm/yr from ∼245 to 230 Ma. The mountain range is a warped extensional footwall, within which white mica cooled from 225 to 205 Ma. Locally, younger extension is recorded by white mica recrystallization at 198–194 Ma, after which the entire block had cooled to below 300°C. Early Cretaceous 40Ar/39Ar ages from the Tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral‐oblique slip along the Xiaotian‐Mozitan fault along the northern boundary. Coeval dextral and sinistral shear zones along the northern and southwestern margins of the Hong'an and Dabie Shan would have caused eastward lateral extrusion of these two blocks, perhaps driven by collision of the Lhasa block with Eurasia.

A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China

Three groups of Mesozoic shoshonitic or high-K calc-alkaline intrusive rocks are identified in Dabieshan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic zone, east-central China and they are related to: (I) slab breakoff; (II) magmatic underplating; and (III) doming. Group-I, the slab breakoff-type, consists of late Triassic (∼210 Ma) mafic monzodiorites. It has moderate to high Sr, and low Rb and Ba abundances, and moderate light rare earth element (LREE)/heavy rare earth element (HREE) and K/Rb ratios. Group-II, the underplating-type, consists mainly of middle Jurassic–early Cretaceous (160–120 Ma) hornblende quartz monzonitic, biotite monzogranitic, and syenogranitic plutons, characterized by relatively high LREE/HREE and K/Rb ratios, and by a large range in concentration of Sr and Ba, coupled with much smaller range in Rb. Group-III, the doming-type, is represented by Cretaceous (125–95 Ma) granitic stocks and granitic porphyry. Compared with group-II, it has high Rb, Y and HREE abundances, low Sr and Ba abundances and low LREE/HREE and K/Rb ratios. All groups have similar Nd and Sr isotopic compositions. Among the three groups, post-collisional granitoid magmatism (group-II) with ages of 160 to 120 Ma, post-dating the HP and UHP metamorphism at 245 to 220 Ma, is the most abundant in the Dabieshan area. The post-collisional granitoid plutons were initially emplaced at different levels ranging from mid-crust to near-surface. This study shows that the whole-rock chemistry of the granitoids vary systematically with crystallization pressures. For example, K 2O, normative Or, Rb and Zr show the strongest increase with decreasing pressure, whereas Ba, Nb, Nd, Yb, MnO, and normative An decrease upward in the Dabie Block. It is suggested that ascent of differentiated, buoyant liquids, combined with fractionation paired with assimilation (AFC), is responsible for the vertical variation. Geological, geochemical and petrological data indicate that group-I could have been generated by partial melting of enriched subcontinental lithosphere mantle due to slab breakoff. Group-II rocks could have been produced mainly from crustal assimilation/melting and fractional crystallization of mantle-derived magmas, whereas group-III magma could have derived from anatexis of the Dabie complex and was highly evolved in a hot doming setting. The late Triassic-early Jurassic slab breakoff may be responsible for the exhumation of UHP rocks through the mantle. The voluminous granitic emplacement together with an episode of rapid denudation suggests that magmatic underplating and inflation could have played a role in the Middle Jurassic–Early Cretaceous rapid exhumation of Dabieshan.

Introduction to geodynamics for high‐ and ultrahigh‐pressure metamorphism

Since the first workshop on ultrahigh‐pressure (UHP) metamorphism at Stanford in 1994 and the special issue of The Island Arc `Ultrahigh‐Pressure Metamorphism and Tectonics' published in December 1995, many symposia and special sessions specifically with regard to the UHP metamorphic terrane have been held. While we are still wondering how exhumation of UHP rocks from mantle depths to the surface takes place, the finding of possible records from the mantle transition zone at 300–400 km depths is astonishing. The study of the UHP regime has expanded to include input from mineral physics, experimental geochemistry and kinetics in addition to the petrochemical and tectonic study of a variety of HP–UHP rocks. It was with this theme that the second workshop for the task group III‐6 `Ultrahigh‐Pressure Metamorphism and Geodynamics in Collision‐type Orogenic Belts' of the International Lithosphere Program was held during the International Geological Congress in Beijing, 1996. The Symposium 8–9 `Dynamic Metamorphic Rocks and High‐ and Ultrahigh‐Pressure Metamorphism' (Cong Bolin & J. G. Liou conveners) had more than 25 presentations in two oral sessions and 70 papers in one poster session. This second special issue of The Island Arc includes nine papers from this symposium and a few related contributions to the geodynamics of HP–UHP metamorphism and tectonics. It is our hope that The Island Arc will continue to publish a special issue on this increasingly recognized subject that is essential to our understanding of continental collision, mantle dynamics and geochemical + fluid cycles.

Exhumation of UHP rocks by transtension in the Western Gneiss Region, Scandinavian Caledonides

Abstract In the Western Gneiss Region (WGR) in the Scandinavian Caledonides, Scandian ecologites ( P = 16 to >28 kbar) occur in a large area of reworked Proterozoic gneisses, structurally below a series of Scandian nappes. The top-to-the-west, extensional Nordfjord-Sogn Detachment (NSD) separates the WGR from allochthonous units, which include several late-orogenic Devonian basins. The allochthon has not experienced Scandian high-pressure (HP) metamorphism. Below the NSD, the WGR is intensely deformed under late-orogenic amphibolite-facies conditions. This deformation is bulk-constrictional, as indicated by a linear feldspar fabric within augen gneisses and tight to isoclinal, lineation-parallel folds within layered gneisses. In a later stage, the NSD, the WGR and the Devonian basins were folded by east-west trending folds, coeval with continuing movement along detachments. To explain these features we propose a transtensional model for the late-orogenic evolution of the WGR. Transtension in West Norway had a sinistral sense and was partially partitioned with increasing transtensional angle towards the NE-SW trending Møre-Trøndelag Fault Zone in the NW. During transtension, there is a strong tendency for rejuvenation of detachments, because detachments fold and may lock as they move. In the WGR, the younger Hornelen Detachment developed above the older NSD. Transtension was the principal exhumation mechanism of the HP and ultra-high-pressure (UHP) rocks in the WGR and involved oblique plate divergence of Laurentia and Baltica during the Early Devonian.

Tectonic evolution of the ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic belts from central China

Abstract In the Triassic suture between the Sino‐Korean and Yangtze cratons, the Dabie metamorphic Complex in central China includes three tectonic units: the northern Dabie migmatitic terrane, the central ultrahigh‐P coesite‐ and diamond‐bearing eclogite belt, and the southern high‐P blueschist‐eclogite belt. This complex is bounded to the north by a north‐dipping normal fault with a Paleozoic accretionary complex and to the south by a north‐dipping reverse fault with Yangtze basement plus its foreland fold‐and‐thrust sequence. Great differences in metamorphic pressure suggests that these units reached different depths during metamorphism and their juxtaposition occurred by wedge extrusion of subducted old continental fragments. These units were subsequently subjected to (i) Barrovian type regional metamorphism and deformation at shallow depths; (ii) intrusion of Cretaceous granitic plutons; and (iii) doming and segmentation into several blocks by normal and strike‐slip faults. A new speculative model of tectonic exhumation of UHP rocks is proposed.