Near-Ultrahigh Pressure Processing of Continental Crust: Miocene Crustal Xenoliths from the Pamir   总被引：8，自引：0，他引：8  

HACKER  BRADLEY; LUFFI  PETER; LUTKOV  VALERY; MINAEV  VLADISLAV; RATSCHBACHER  LOTHAR; PLANK  TERRY; DUCEA  MIHAI; PATINO-DOUCE  ALBERTO; McWILLIAMS  MICHAEL; METCALF  JIM 《Journal of Petrology》2005,46(8):1661-1687

Xenoliths of subducted crustal origin hosted by Miocene ultrapotassicigneous rocks in the southern Pamir provide important new informationregarding the geological processes accompanying tectonism duringthe Indo-Eurasian collision. Four types have been studied: sanidineeclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite),felsic granulites (garnet, quartz, sanidine and kyanite), basalticeclogites (omphacite and garnet), and a glimmerite (biotite,clinopyroxene and sanidine). Apatite, rutile and carbonate arethe most abundant minor phases. Hydrous phases (biotite andphengite in felsic granulites and basaltic eclogites, amphibolesin mafic and sanidine eclogites) and plagioclase form minorinclusions in garnet or kyanite. Solid-phase thermobarometryreveals recrystallization at mainly ultrahigh temperatures of1000–1100°C and near-ultrahigh pressures of 2·5–2·8GPa. Textures, parageneses and mineral compositions suggestderivation of the xenoliths from subducted basaltic, tonaliticand pelitic crust that experienced high-pressure dehydrationmelting, K-rich metasomatism, and solid-state re-equilibration.The timing of these processes is constrained by zircon agesfrom the xenoliths and ⁴⁰Ar/³⁹Ar ages of the host volcanic rocksto 57–11 Ma. These xenoliths reveal that deeply subductedcrust may undergo extensive dehydration-driven partial melting,density-driven differentiation and disaggregation, and sequestrationwithin the mantle. These processes may also contribute to thealkaline volcanism observed in continent-collision zones. KEY WORDS: xenolith; high-pressure; subduction; Pamir; Tibet  相似文献   

Metamorphism, geochemistry and origin of magnesian volcanic rocks, Klamath Mountains, California     

B. R. HACKER  W. G. ERNST  M. D. BARTON 《Journal of Metamorphic Geology》1992,10(1):55-69

Metabasaltic rocks in the Klamath Mountains of California with ‘komatiitic’ major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentrations preserve relict igneous trends and suggest that the rocks are not komatitic basalts, but immature arc rocks and within-plate alkalic lavas. Correlation of ‘excess’ MgO with the volume per cent hornblende (±clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO-rich compositions. Early submarine alteration produced regional δ¹⁸O values of +10±1.5%_° and shifts in Al₂O₃, Na₂O, and K₂O concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschist facies (350–550°C, c. 3 kbar) southward to prehnite–actinolite facies (200–400°C, ≤3 kbar), but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclase + microcline + chlorite + titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these phases biotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearly stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatible with P–T conditions in a magmatic arc. Later contact metamorphism at 2–2.9±0.5 kbar and at peak temperatures approaching 600° C around the English Peak and Russian Peak granodiorites produced 3–4–km-wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, muscovite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatism during contact metamorphism produced further increases in bulk-rock δ¹⁸O_SMOW of as much as +6%_°. Thus, the unusually MgO-rich nature of the Sawyers Bar rocks may be attributed at least partly to metasomatism and the presence of magnesian cumulus phases.  相似文献   

The thermal structure of continental crust in active orogens: insight from Miocene eclogite and granulite xenoliths of the Pamir Mountains     

S. M. GORDON  P. LUFFI  B. HACKER  J. VALLEY  M. SPICUZZA  R. KOZDON  P. KELEMEN  L. RATSHBACHER  V. MINAEV 《Journal of Metamorphic Geology》2012,30(4):413-434

Rare ultrahigh‐temperature–(near)ultrahigh‐pressure (UHT–near‐UHP) crustal xenoliths erupted at 11 Ma in the Pamir Mountains, southeastern Tajikistan, preserve a compositional and thermal record at mantle depths of crustal material subducted beneath the largest collisional orogen on Earth. A combination of oxygen‐isotope thermometry, major‐element thermobarometry and pseudosection analysis reveals that, prior to eruption, the xenoliths partially equilibrated at conditions ranging from 815 °C at 19 kbar to 1100 °C at 27 kbar for eclogites and granulites, and 884 °C at 20 kbar to 1012 °C at 33 kbar for garnet–phlogopite websterites. To reach these conditions, the eclogites and granulites must have undergone mica‐dehydration melting. The extraction depths exceed the present‐day Pamir Moho at ～65 km depth and suggest an average thermal gradient of ～12–13 °C km^?1. The relatively cold geotherm implies the introduction of these rocks to mantle depths by subduction or gravitational foundering (transient crustal drip). The xenoliths provide a window into a part of the orogenic history in which crustal material reached UHT–(U)HP conditions, partially melted, and then decompressed, without being overprinted by the later post‐thermal relaxation history.  相似文献   

Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation   总被引：1，自引：0，他引：1  

D. B. ROOT  B. R. HACKER  P. B. GANS  M. N. DUCEA  E. A. EIDE  J. L. MOSENFELDER 《Journal of Metamorphic Geology》2005,23(1):45-61

New eclogite localities and new ⁴⁰Ar/³⁹Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km²; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. ⁴⁰Ar/³⁹Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New ⁴⁰Ar/³⁹Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. ⁴⁰Ar/³⁹Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr^?1), perhaps because of continued tectonic unroofing.  相似文献   

⁴⁰Ar/³⁹Ar Constraints on the tectonic history and architecture of the ultrahigh-pressure Sulu orogen     

B. R. HACKER  S. R. WALLIS  M. O. MCWILLIAMS  P. B. GANS 《Journal of Metamorphic Geology》2009,27(9):827-844

New ⁴⁰Ar/³⁹Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar ⁴⁰Ar/³⁹Ar ages. Subsequent unroofing at rates of 5–25 km Myr⁻¹ brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by ⁴⁰Ar/³⁹Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr⁻¹. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr⁻¹, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr⁻¹, suggesting relatively shallow crustal depths.  相似文献