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Inclusions of polycrystalline K-feldspar aggregates after K-cymrite (KAl[Si.sub.3][O.sub.8] x n[H.sub.2]O) were discovered in garnet from a Dulan eclogite in the Qaidam ultrahigh-pressure (UHP) terrane, western China. The eclogite consists of garnet ([Alm.sub.56][Grs.sub.23][Prp.sub.20][Sps.sub.01]), omphacite ([Jd.sub.35][Aeg.sub.6][Aug.sub.59]), and minor ruffle and apatite. The 20 to 200 [micro]m inclusions vary in shape from prismatic, hexagonal to rounded, and exhibit palisade and mosaic textures. Host garnets show radial fractures, similar to those surrounding quartz pseudomorphs after coesite. Some inclusions consist of almost end-member K-feldspar ([Or.sub.99-100][Ab.sub.0-1]) polycrystalline aggregates, whereas others are composed of >90 vol% K-feldspar ([Or.sub.96-99][Ab.sub.1-4]) with minor secondary albite occurring along the margins of the inclusions. Raman spectra of K-feldspar crystalline aggregates vary slightly reflecting various degrees of Si-Al ordering, and show a Raman peak at ~390-395 [cm.sup.-1], typical for cymrite structure. These characteristics of the K-feldspar polycrystalline inclusions imply the presence of former K-cymrite in the Dulan eclogite formed at >3 GPa at ~720 [degrees]C. The occurrence of K-cymrite in UHP eclogite is significant because of its potential as an important carrier of crustal K and [H.sub.2]O to the upper mantle. Keywords: K-cymrite, K-feldspar, inclusion, eclogite, Raman spectrum, North Qaidam UHP terrane

Kosmochloric diopside with high K content up to 0.56 wt% (0.026 K atom per formula unit) was discovered from the Osayama serpentinite melange in the Chugoku Mountains, SW Japan. K-bearing clinopyroxene fills microcracks (5-150 [micro]m in wide) together with uvarovite within albite vein of a tremolite rock. Compositions of analyzed clinopyroxene consist mainly of kosmochlor + augite (92-98 mol%; [K[o.sub.19-38][Aug.sub.56-76]) components and minor amounts of jadeite (0-6 mol%), aegirine (0-5 mol%), Ca-Tschermak (0-3 mol%), and K-kosmochlor (0-2 mol%). Although the K content in clinopyroxene is also variable and heterogeneous even in a single vein, clinopyroxene with higher K content occurs in Ko-rich part. Higher magnification secondary electron images confirmed that exsolution and inclusion are essentially absent in the analyzed clinopyroxenes. The good negative correlation between Cr + Na + K and Ca + Mg + F[e.sup.2+] indicates the Cr incorporation into the octahedral site. Furthermore, K correlates with Na and Cr, indicating a simultaneous enrichment of K for Na and Cr during pyroxene growth. Textual relations, and parageneses and compositions of minerals suggest that the K-Cpx precipitated together with uvarovite in brittle microcracks directly from a Ca- and Cr-rich hydrothermal fluid at approximately P < 0.3 GPa and T< 400 [degrees]C. Although it has been experimentally concluded that only ultrahigh-P (>4 GPa) environment permits to host relatively large [K.sup.+] cation into the clinopyroxene structure, our finding indicates that the incorporation of K into the kosmochlor-diopside series solid solution with at least 0.2 Cr cation p.f.u. is possible even at low P conditions.

Coexisting jadeite and omphacite were found as retrograde minerals in a jadeite-bearing lawsonite-eclogite from the Motagua Fault Zone, Guatemala. The lawsonite-eclogite is characterized by the occurrence of garnet porphyroblasts up to 2.5 cm in size, and the eclogite-facies parageneses, almandine-rich garnet + impure jadeite + lawsonite + rutile + quartz; garnet contains inclusions of impure jadeite, phengite, ferroglaucophane, chlorite, lawsonite, rutile, ilmenite, and quartz. Textural relations and parageneses and compositions of minerals indicate that the lawsonite-eclogite experienced two stages of metamorphism: prograde eclogite-facies stage ([M.sub.1]) and retrograde stage ([M.sub.2]). The impure jadeite (Jd-I) of the [M.sub.1] eclogite-facies occurs in both the matrix and as inclusions in garnet, and contains considerable amounts of augite and aegirine components ([Jd.sub.61-75][Aug.sub.16-24][Ae.sub.0-18]). It is partly recrystallized to retrograde [M.sub.2] jadeite (Jd-II) ([Jd.sub.74-87][Aug.sub.9-16][Ae.sub.0-11]) and omphacite ([Jd.sub.42-50][Aug.sub.36-46][Ae.sub.7-16]); some of these two sodic pyroxenes may have crystallized from fluids. Both [M.sub.2] jadeite and omphacite show textural equilibrium and are believed to have grown concurrently. Based on the observed compositions and the phase relations of sodic pyroxenes from Carpenter (1980), the [M.sub.1] impure jadeite (Jd-I) may have had a disordered C2/c symmetry at T = ca. 450 [degrees]C and P = ca. 1.8-2.4 GPa, and was subsequently crystallized into jadeite (Jd-II) plus ordered P2/n omphacite during retrogression with infiltration of fluids at T< ca. 300 [degrees]C and P = ca. 0.7 GPa ([M.sub.2]). The extreme low-T conditions during retrogression may have prevented reaction between eclogitic jadeite and adjacent minerals. Instead, eclogitic impure jadeite (plus fluid) has recrystallized into the retrograde jadeite + omphacite pair with a wide compositional gap.

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSD--PP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China Many low-P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low-P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to coesite. The 227 [+ or -] 9 Ma (the weighted mean age) obtained from coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses.

The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet lherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered stRucture from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition ([Prp.sub.64-67][Alm.sub.23-24][Grs.sub.10-12][Sps.sub.1]). Garnet (PrpsI-63) from clinopyroxenite contains more pyrope component than eclogitic garnets ([Prp.sub.29-35]). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of futile and apatite ([+ or -]Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower [[delta].sup.18][O.sub.VSMOW] of +4.40% for olivine, O-isotopic compositions (+5.03 to +5.53%) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle [[delta].sup.18]O values of +5.1 to +6.6%0. Small O-isotope fractionations ([DELTA]Grt-Cpx: -0.16 to -0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 [+ or -] 50 [degrees]C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

Two types of amphibole exsolution lamellae were discovered in garnet from a few garnet peridotites from the North Qaidam ultrahigh-pressure metamorphic belt, northern Tibet, NW China. The amphibole lamellae are strictly oriented in four directions corresponding to the isometric form {111} (i.e., octahedron planes) of garnet. Observations by transmission electron microscopy (TEM) show that amphibole lamellae are topotaxtially concordant with the host garnet and possess 9.8 to 10 lattice 'c'-spacing. Electron-microprobe analysis reveals that these exsolved amphibole lamellae are high in Na and Ti. The TEM observations and recalculated compositions indicate that the oriented amphibole lamellae exsolved from original supersilicic majorites with high concentrations of [Na.sub.2]O (0.3 wt%) and hydroxyl (up to 1000 ppm by weight). These results imply that the host garnet peridotites were formed at depths greater than 200 km and that garnet can be an important reservoir of water at such depths in the mantle.

The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm [H.sub.2]O. The silica phases surrounding coesite, however, show varying amounts of [H.sub.2]O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (< 100 ppm [H.sub.2]O, red-violet CL); 'mosaic' quartz, which is actually chalcedony (up to 0.4 wt% [H.sub.2]O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% [H.sub.2]O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm [H.sub.2]O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 [micro]m long and 2-3 [micro]m wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 [cm.sup.-1], typical for coesite, in addition to those of quartz and the host omphacite. Such occurrences together with textured observations suggest a two-stage evolution of Si[O.sub.2] exsolution rods in omphacite. Lamellae of coesite were apparently exsolved from supersilicic omphacite at P [approximately equal to] 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.

Eclogites interlayered with gneiss, and minor quartzite in the Qinglongshan near Donghai are characterized by unusuall abundant (15-40 vol%) hydrous phases including talc. phengite, and epidote; many also contain kyanite. Garnet hosts, both prograde (paragonite, amphibole, epidote) and peak stage (omphacite, epidote, phengite, kyanite) mineral inclusions. Several eclogites contain talc rimmed by barroisite: optically and compositionally similar coarse-grained amphibole in other samples indicates that the reaction Omp + Tlc = Amp has completely consumed tale. Estimated peak conditions of 30-35 kbar, 600-700 [degrees]C. are consistent with polycrystalline quartz pseudomorphs after coesite included in garnet, omphacite, epidote, and kyanite, and up to 3.6 Si pfu (11 O atom basis) in phengite. Garnet-epidote oxygen barometry on the peak metamorphic assemblage indicates oxygen fugacities above the Hem-Mag buffer, consistent with the epidote + talc assemblage and 5-20 mol% aegerine component in omphacite. The high oxygen fugacity calculated in this study as well as previously documented negative oxygen isotope values recorded by these rocks may both reflect alteration by oxidizing, meteoric water in a hydrothermal system. Oxidized conditions during peak metamorphism may explain the extreme scarcity of microdiamond in this area. The Ep + Tlc assemblage is stabilized by high oxygen fugacity, and demonstrates that talc-bearing eclogites are not restricted only to unusually Mg-rich bulk composition.

Chromian clinopyroxenes with exsolution textures were found in high-pressure (HP) tremolite schist from the Osayama serpentinite melange in the Chugoku Mountains, Southwestern Japan. Chromian omphacite [jadeite [(Jd).sub.23.3-41.4] diopside [(Di).sub.46.7-.60.7] kosmochlor [(Ko).sub.4.5-19.6] aegirine [(Ae).sub. Chromian pyroxenes replacing relict chromian spinel are associated with other chromian silicates including phengite, chlorite, and pumpellyite. The wide compositional gap of chromian pyroxenes and the presence of chromian pumpellyite and Si-rich chromian phengite indicate T < 300-400 [degrees]C and P > 0.8 GPa. This P-T estimate is consistent with parageneses of minerals in the host serpentinite. The variation of Cr content in chromian silicates reflects the extent of Cr [left and right arrow] A1 substitution, and may be related to a chemical heterogeneity of the Cr-bearing fluid.

The Rizhao garnet clinopyroxenite occurs as small mantle slices faulted into subducted continental crust in the Sulu ultrahigh-pressure (UHP) terrane, China. Two representative garnet clinopyroxenites with exsolation microstructures were studied: (1) a massive ilmenite-rich specimen, composed of coarse-grained clinopyroxene containing abundant exsolution rods of grossular-rich garnet (>20 vol%) + ilmenite (~3-4 vol%) in a fine-grained matrix of clinopyroxene + garnet + ilmenite; and (2) megacrystic garnet-bearing sample, characterized by garnet containing inclusions of clinopyroxene and ilmenite; the clinopyroxene inclusions show exsolution rods and blebs with variable amounts of garnet (1-15 vol%) and minor ilmenite. Both exsolved and matrix garnets have similar grossular-rich compositions. The aggregate composition of clinopyroxene + garnet + ilmenite intergrowth is similar to the whole-rock composition. We propose that the parental phase of exsolved garnet + ilmenite lamellae and clinopyroxene host was either a homogeneous clinopyroxene (hypothesis A) or a majoritic garnet (hypothesis B) that experienced three discrete evolution stages. If the parental phase was clinopyroxene (AB[O.sub.3]), with an Si deficiency in the B site, the Rizhao garnet clinopyroxenite could have an initial assemblage clinopyroxene [+ or -] Grt [+ or -] Ilm formed at near-solidus conditions ([less than or equal to]1400[degrees]C, at 5 GPa) in the upper mantle. The second stage is defined by garnet exsolution from the primary clinopyroxene involving decreasing temperature and/or increasing pressure, related to continental subduction. Coexisting clinopyroxene host and garnet exsolution ([Grt.sub.exs]) recrystallized at temperatures of ~900[degrees]C at an assumed minimum P of 5 GPa. The third stage is represented by recrystallization of exsolution phases to form the fine-grained matrix of Cpx + Grt + Ilm at 700[degrees]C and [is greater than or equal to] 3 GPa during the early exhumation of the UHP terrane. The speculative hypothesis B implies that the proposed majoritic garnet was from the mantle transition zone (

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554-494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite-bearing garnet peridotites. Keywords: mantle metasomatism, Ti-clinohumite, zircon, U-Pb geochronology, ultrahigh-pressure metamorphism.

Transmission electron microscopy (TEM) reveals that some enstatites from garnet-pyroxenites in the Chinese Dabie-Sulu ultrahigh-pressure (UHP) terrane consist of intimate intergrowths of orthoenstatite (OREN) and clinoenstatite (CLEN), and some grains of pyroxene (MgFe)[.sub.2][Si.sub.2][O.sub.6] exhibit inclined extinction. These garnet pyroxenites are enclosed in coesite-bearing eclogite or interlayered with harzburgite and omphacitite. They contain garnet ([Prp.sub.54] to [Prp.sub.73]), 'enstatite' (En85 to [En.sub.92]), magnesite, and titanomagnetite (Sulu sample) or clinohumite (Dabie sample), and recrystallized at about 4.0-6.5 [+ or -] 0.2 GPa, ~ N750 [+ or -] 50[degrees]C. All clinoenstatite lamellae in the host orthoenstatite have even numbers of 9 [Angstrom] (100) fringes without twins, and are oriented parallel to (100) of the host. The lamellae and host phases have nearly identical (MgFe)[.sub.2][Si.sub.2][O.sub.6] compositions. The thickest observed CLEN lamellae are -0.4 gm. Clinoenstatites from Sulu have cell parameters: a = 9.67(7) [Angstrom], b = 8.88(6) [Angstrom], c = 5.22(3) [Angstrom], [beta] = 107.7(2)[degrees], and V = 427(8) [[Angstrom].sub.3], and those from Dabie: a = 9.53(6) [Angstrom], b = 8.88(6) [Angstrom], c = 5.24(3) [Angstrom], [beta] = 107.0(2)[degrees], and V = 424(8) [[Angstrom].sub.3]. The TEM electron-diffraction patterns indicate that both clinoenstatites possess P[2.sub.1]/c structures and are the low-P polymorph of clinoenstatite. The host orthoenstatites have cell dimension: a = 18.32(12) [Angstrom], b = 8.88(6) [Angstrom], c = 5.20(3) [Angstrom] for the Sulu sample, and a = 18.24 (12) [Angstrom], b = 8.88(6) [Angstrom], c = 5.18(3) [Angstrom] for the Dabie sample. Clinoenstatites in garnet pyroxenites from the Dabie-Sulu UHP terrane may have formed either by inversion from OREN, or by the displacive transformation from high-P clinoenstatite during decompression.

Hydroxyl-rich topaz was recognized in thick kyanite quartzites from both high-pressure (HP) and ultrahigh-pressure (UHP) belts of the Sulu terrane, China. These quartzites contain variable proportions of quartz, kyanite, and topaz, with minor phengite, pyrite, and futile. Some topaz grains from the UHP belt contain abundant inclusions of oriented kyanite, whereas those from the HP belt are partially replaced by woodhouseite [Ca[Al.sub.3](P[O.sub.4])(S[O.sub.4])[(OH).sub.6]]. Most topaz crystals contain 9.5 to 13.5 wt% F [0.92-1.30 atoms per formula unit (apfu)], indicating 35-55% substitution of F by OH. Such naturally occurring, hydroxyl-rich topaz has not been previously reported. Some topaz grains from the HP belt show distinct zoning: (1) decreasing F content from narrow cores (13.3-16.5 wt%) to thick rims (9.5 wt%) or (2) oscillatory zoning (9.44-12.77 wt%). Unit-cell parameters of topaz show a positive linear correlation between the OH content and a and b as well as volume. Based on our petrologic data, the experimentally determined curve of Ky + [H.sub.2]O = topaz-OH at very high pressures, and calculated OH/(OH + F) isopleths of topaz at low pressures, the topaz with [X.sub.OH] ~0.35 from the UHP belt may have formed at P-T conditions within the coesite stability field. The P-T condition of HP topaz is less well constrained; its high [X.sub.OH] (0.40-0.55) may have been caused by lower metamorphic temperatures and higher initial [X.sub.OH] in comparison with UHP topaz. Hydroxylrich topaz together with other hydrous minerals in UHP rocks may be important carriers of [H.sub.2]O to mantle depths of 100-200 km during continental subduction.

Ultrahigh-pressure (UHP) minerals of the Maowu mafic-ultramafic complex in the Dabieshan, east-central China exhibit many exsolution textures. Magnetite lamellae are common in olivines and Ti clinohumites from harzburgite and garnet pyroxenite. Monazite-(Ce) lamellae occur in apatites from the garnet pyroxenite. Independent P-T estimates suggest that these ultramafic rocks formed at P > 5 GPa and T = 700 [+ or -] 50 [degrees] C. The lamellae-bearing minerals are believed to preserve an earlier, higher P-T record prior to exsolution. Compositions and unit-cell parameters of the magnetite and host olivine and intergrowth relations were determined using a newly developed X-ray diffraction microprobe technique employing synchrotron radiation. The host olivine and magnetite lamellae bear a topotaxial relation with [[220].sub.Mag] || [[200].sub.Ol], [[111].sub.Mag] || [Mathematical Expression Omitted], [Mathematical Expression Omitted] || [[331].sub.Ol], [[242].sub.Mag] || [Mathematical Expression Omitted]. The recalculated composition of primary olivine may contain up to 1.5 wt% [Fe.sub.2][O.sub.3]. Four hypotheses may explain the observed intergrowths of oriented magnetite lamellae in olivine: (1) oxidation of olivine; (2) decomposition of [Fe.sup.3+]-bearing olivine formed at >6 GPa; (3) exsolution of a spinel (wadsleyite) solid-solution [Fe.sub.3][O.sub.4]-[(Fe,Mg).sub.2]Si[O.sub.4] during decompression; and (4) breakdown of phase A [[Mg.sub.7][Si.sub.2][(OH).sub.6]] + enstatite. The third hypothesis appears to be the most likely inasmuch as no additional silicate phase occurs as associated inclusions in the olivine host. However, the actual mechanism for exsolution of magnetite from olivine remains to be studied experimentally.

The northeastern extension of the Altyn Tagh fault, China, beyond the apparently truncated Qilian Shan has been controversial since the recognition of this large left-slip fault. We propose that the Altyn Tagh fault may have been contiguous with the east-northeast to northeast-trending, currently inactive (but apparently active during the Cenozoic) Alxa-East Mongolia fault; the latter truncated the Beishan orogen to the northwest and the Inner Mongolia orogen to the southeast. The Beishan orogen can be correlated with the Inner Mongolia orogen on the basis of the recognition of three offset petrotectonic units and two offset sutures. This correlation requires 400 [+ or -] 50 km of left-lateral offset along the Alxa-East Mongolia fault, identical to the slip along the northern segment of the Altyn Tagh fault inferred by P. Molnar and P. Tapponnier. We suggest a two-stage model for fault evolution: during the first stage, 400 km of displacement separated the Inner Mongolia orogen from the originally contiguous Beishan orogen along a continuous Altyn Tagh-Alxa-East Mongolia fault. During the second stage, the Alxa-East Mongolia fault became inactive, and additional offset along the Altyn Tagh fault has been mainly accommodated by shortening of the Qilian Shah and the Qaidam basin. We infer that the first stage of movement along the Altyn Tagh-Alxa-East Mongolia fault system may have begun around Oligocene time: the region transformed to the second stage of fault movement ca. 13-16 Ma, probably because of thinning of the lithospheric mantle beneath northern Tibet. The proposed model suggests that the Derbur fault in northernmost China and beyond is an extension of the Alxa-East Mongolia limit, and has a history of strike-slip movement. The nearly 400 km of offset on the Alxa-East Mongolia fault may have been accommodated mainly by subduction in the Sea of Okhotsk.

New 40Ar/39Ar ages from rocks in the high-pressure and ultra-high-pressure (HP-UHP) metamorphic complex in the Hong'an and Dabie Mountains areas, east-central China, document a two-phase cooling and exhumation history following Triassic continental collision and metamorphism. Phengite ages from blueschist through kyanite-bearing eclogite facies rocks in Hong'an record initial exhumation from the collision zone between 230 and 195 Ma. Biotite and horn-blende ages from migmatites and eclogite-bearing gneisses from the Dabie Mountains record a cooling event between 128 and 117 Ma, corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons. The Triassic through Early Jurassic 40Ar/39Ar cooling ages corroborate U-Pb and Sm-Nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid. Emplacement of granitic melts within the HP-UHP sequences [approximately] 80 m.y. after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial, rapid exhumation episode.

The largest gabbroic intrusion, Jiaoziyan, in the Northern Orthogneiss of the Dabie Shan was investigated to test whether such intrusions represent arc-related or collision-related magma, as postulated in recent regional tectonic models. The Jiaoziyan intrusions comprise a gabbro-monzogabbro suite and are locally characterized by cumulus textured anhydrous silicates. Petrologic indications against arc magmatism include (1) coexisting olivine-plagioclase compositions beyond the documented range of arc gabbros, and (2) a compositional trend of Al/Ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction-related gabbros worldwide. In contrast, the Jiaoziyan rocks are mineralogically similar to rift gabbros. Geochronologic and geometric arguments against collision-related magmatism (i.e., as a consequence of slab breakoff) include the following: (1) crystallization of the Jiaoziyan mafic intrusion postdated the Triassic (245-240 Ma) subduction and peak orogenic metamorphism by ~100 m.y. and (2) the rocks of the Dabie Cretaceous magmatic suite, including Jiaoziyan, are on the footwall plate, rather than on the hanging wall, of the Triassic subduction-zone framework and do not show a linear distribution. These observations argue that the voluminous Early Cretaceous magmatism in the Northern Orthogneiss is geodynamically unrelated to the Qinling-Dabie orogeny, but was caused by extensional tectonism related to regional magmatic under-plating beneath the thinned Dabie crust. Keywords: gabbros, petrogenesis, extension tectonics, Cretaceous, Dabie Mountains.

Mineral assemblages and microstructures of a newly identified (retrogressed) eclogite (sensu lato) in the North Dabie Complex (NDC), central-eastern China, indicate early very high-pressure metamorphic relics preserved in a dominant amphibolite-facies host, where no eclogite (sensu stricto) has been positively identified before. The investigated eclogitic rock shows distinct multistage recrystallization, with granulite- and amphibolite-facies assemblages overprinting eclogite-facies relics. The minimum temperature for the eclogite-facies metamorphism is estimated to be ~800-820 [degrees] C. A spectacular microstructure of oriented quartz needles (~2-20 [micro]m wide, ~5-200 [micro]m long) in matrix Ca-Na clinopyroxene implies the prior existence of a non-stoichiometric 'supersilicic' omphacite stabilized at ultrahigh-pressure (UHP, [is greater than or equal to] 25 kbar) conditions, although no coesite or coesite pseudomorphs have been found in the samples. The absence of coesite may be due to the lack of free silica at UHP conditions or the consumption of silica during retrograde reactions. The inferred UHP conditions metamorphism is further supported by Sm-Nd ages that are equivalent to Triassic metamorphic ages from UHP eclogites in the southeastern Dabie Mountains. This finding expands the UHP terrane northward about 50 km; spatial distribution of subduction/collision-related UHP rocks includes parts of the NDC.