Mineral evolution of a garnet-pyroxenite nodule within eclogite, eastern Sulu ultrahigh-pressure metamorphic terrane, East China

Detailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.     

海南文昌二辉橄榄岩中辉石出溶结构的结晶学取向分析

本文用电子背散射衍射技术(electron backscatter diffraction,EBSD)测试了海南文昌玄武岩中二辉橄榄岩包体中的辉石主晶与其出溶片晶的结晶学取向关系。结合电子探针成分测试,得出:单斜辉石(透辉石)主晶中出溶了两组不同方向的片晶,一组为斜方辉石(顽火辉石-易变辉石)片晶,另一组为单斜辉石(普通辉石)片晶。由于出溶片晶在EBSD测试切面上体现为以线状体,因此需要找到一种方法将线状体所代表的片晶的晶面符号推算出来。本文介绍了一种利用吴氏网进行坐标系旋转的"晶带相交法",该方法可以作图推算出溶片晶的晶面符号。根据"晶带相交法"得出,斜方辉石(顽火辉石-易变辉石)出溶片晶为(100),单斜辉石(普通辉石)出溶片晶为~(401)。根据前人的研究资料,出溶片晶~(401)可能指示最小出溶压力为9.5~12.5GPa。出溶片晶的结晶学取向涉及到主晶与出溶体的晶体结构匹配关系,并且与出溶温度-压力有关,因此出溶片晶的结晶学取向分析具有晶体化学理论意义和反映地质温压过程的实际意义。这种"晶带相交法"可以推广应用于其他矿物出溶结构的结晶学取向研究中。     

Microstructural defects in experimentally shocked diopside: A TEM characterization

Transmission electron microscopy was used for characterizing the defect microstructure induced by shock experiments in a single crystal of diopside. The shock-induced defects found in the crystal can be divided in four distinct types:

1. A high density and pervasive distribution of dislocations in glide configuration (glide systems (100)[0
2. Mechanical twin lamellae, mostly parallel to (100), the (001) twin lamellae are less abundant. li]3)
3. Heterogeneously distributed tiny molten zones (3 to 20 μm size) which, after cooling, appear as a glass with a chemical composition very close to the one of the original diopside.

The present TEM study reveals that the defect micro-structure in shocked diopside consists of a large variety of shock-induced defects. Especially, the amorphous PDFs which were never observed in statically deformed diopside seem to be an important characteristic micro-structural defects in shocked silicate minerals. Although the presence of amorphous PDFs is not yet confirmed for naturally shocked clinopyroxene, we strongly suggest that these features can serve as a diagnostic tool for recognizing impact phenomena on all planetary bodies of our solar system.     

Texture and structural interpretation of the alteration of pyroxene to other biopyriboles

In a metamorphosed gabbro from Hokkaido, Japan, augite containing exsolved orthohypersthene and minor pigeonite has been altered to a variety of biopyriboles. High resolution transmission electron microscopy of slightly altered augite shows only narrow (010) lamellae of clinoamphibole which always contain even numbers of double chains. In more highly altered regions, all three pyroxenes are changed to double-, triple- and more highly polymerized multiple-chain biopyriboles, with chlorite d ₀₀₁ 14.5 Å) found only in orthopyroxene. Several (010) lamellae containing only one double chain have been observed, and their textural relationship to the surrounding single-chain host may explain how the rotated domains of biopyribole initially attain their orientation in the host pyroxene.A structural model is proposed for the polymerization of single chains in the tetrahedral layers of pyroxene which involves small movements of oxygens and tetrahedral atoms (Si, Al) with a minimum of bond breaking and re-forming, concommitant with hydroxylization of certain oxygens and the diffusion of Mg, Fe and/or Ca along the rift in the octahedral layer.     

The lamellar structure in moonstone and anorthoclase from Korea

Moonstone and anorthoclase from Korea were investigated using electron microscope. Lamellae of moonstone were in the range of 1,200 to 3,000 Å, and anorthoclase have fine lamellae ranging from 20 to 900 Å. The outer part of every specimen has exsolved more perfectly, and the lamellae are coarse. The inner part is in the process of exsolution, and the lamellae are extremely fine. The lamellae are parallel to (¯801), and the streations on diffraction spots are oriented normal to the lamellae.     

An AEM study of garnet clinopyroxenite from the Sulu ultrahigh-pressure terrane: formation mechanisms of oriented ilmenite,spinel, magnetite,amphibole and garnet inclusions in clinopyroxene

Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [1[`1]00 1\bar{1}00 ]<sub>Ilm</sub>//[0[`1]0 0\bar{1}0 ]_Cpx (0001)_Ilm//(100)_Cpx; and [110]_Spl//[0[`1]0 0\bar{1}0 ]<sub>Cpx</sub> ([`1]11 \bar{1}11 )_Spl//(100)_Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness <1 μm) and lamellae (with a thickness = 1–10 μm) in host clinopyroxene of sample RZ-11D were probably results of hydration processes, although amphibole plates could otherwise be interpreted as exsolution products. Temporal relations between mineral inclusions in each sample can be established, and a semi-quantitative P–T path for this garnet clinopyroxenite body was derived accordingly. The present results show that the Hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during UHP metamorphism as suggested by previous studies. This study demonstrates that the crystallographic and temporal/spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks.     

The behavior of protons during wet olivine deformation under the conditions of the kimberlite process

This paper reports the results of optical and electron microscopic investigations of mantle olivine samples with H₂O contents of tens-hundreds ppm weight. Samples were obtained from the xenoliths and xenocrysts of the Udachnaya pipe. At the scale of optical microscope magnification, a peculiar banded microstructure was observed in thin sections prepared parallel to the olivine (010) plane. It is formed by cross-hatched bands parallel to four crystallographic directions of the olivine structure: [100], [001], [101], and [−101]. At the scale of electron optical magnifications, the banded microstructure is observed as nanometer-sized heterogeneities of various types which are related to olivine deformation: (a) planar defects parallel to (100) and (001) corresponding to the (100)[010] and (001)[100] dislocation glide systems, respectively; they are occasionally transformed into lamellae or decorated by nanoinclusions; and (b) nanometer-sized heterogeneities formed by nanoinclusion arrays not related to planar defects and oriented along the same directions of the olivine structure as the optically visible bands. The deformation structures are decorated by coupled point OH-bearing defects, which were initially present in the olivine. The crystallographically oriented arrays of nanoinclusions of high-pressure hydrous silicates are considered as a result of olivine deprotonization (elimination of OH-bearing defects from the olivine structure) in the zones of previous deformation compression in the crystal. Light refraction effects on the nanoinclusions make these zones optically visible and produce the banded microstructure, which is a consequence of previous deformation.     

Submicroscopic exsolution lamellae in pyroxenes in the Whin Sill,Northern England

Optically homogeneous pigeonites and augites from Whin Sill dolerite cores from Throckley (Northumberland) are shown by electron microscopy to be unmixed. The lamellae are 40 Å wide at the margins of the sill and up to 3200 Å wide at the centre. Homogeneous pyroxenes also occur with a composition intermediate between the pigeonite and augite. Electron diffraction patterns of the unmixed grains show that the augite contains pigeonite lamellae and the pigeonite contains augite lamellae. From the application of simple diffusion theory it is suggested that the size of the lamellae is dependent on the rate of cooling of the sill.     

The Subsolidus History of the Artfj?llet Gabbro: a TEM Study of Olivine, Augite and Orthopyroxene

A transmission electron microscope study of the microstructuresof olivine, augite and ortho-pyroxene in the Artfj?llet gabbroshows that the gabbro has been affected by two phases of deformation,exsolution and alteration. During the first phase, deformationand annealing of olivine led to the formation of elongated subgrains,with (100) subgrain boundaries consisting of regularly spacededge dislocations with [100] Burgers vectors. Localized strongerdeformation resulted in the development of mosaic subgrainswith (100) and (001) boundaries. Exsolution produced blebs and(100) lamellae of orthopyroxene in augite. Two types of blebsoccur, referred to as symplectitic augite and blebby augite.Symplectitic augite formed through discontinuous precipitation.The blebs in blebby augite are considered to have nucleatedat subgrain boundaries or dislocations in the augite. Blebs,melon pips and (100) lamellae of augite formed in orthopyroxene.These blebs and melon pips are thought to be due to nucleationat dislocations or to thickening of (100) lamellae at dislocations.Brown hornblende formed as blebs in augite and to a lesser extentorthopyroxene. This first event took place while the gabbrowas at a temperature of c. 900-1000 ?C and is thought to berelated to D_2b, a regional flattening and thrusting event. Renewed exsolution in the pyroxenes occurred later, probablybefore the second subsolidus phase, -but its timing is poorlyconstrained. "100" lamellae of low-Ca clinopyroxene formed inaugite at an estimated temperature of c. 600 ?C. Thin (100)lamellae and Guinier-Preston zones were developed in orthopyroxene. During the second phase, deformation resulted mainly in cracks,along which hydrous fluids entered the gabbro, causing a variabledegree of metamorphism at a temperature of c. 500-550 ?C. Inolivine irregular dislocations of mixed edge-screw characterformed. Cr-spinel with an unknown phase, or magnetite with diopsideprecipitated in the olivine. Oxide minerals formed in the pyroxenesand brown hornblende: Cr-spinel in Cr-rich augite; rutile andilmenite in other augite, orthopyroxene and brown hornblende.The formation of these oxide minerals is thought to be due toexsolution of cations such as Ti and Cr, which these mineralsdo not tolerate in their structures at low temperatures, combinedwith oxidation in the case of magnetite in olivine. Alterationresulted in thin amphibole lamellae parallel to (010) in augite.This second subsolidus phase is correlated with the D₃ regionaldeformation phase and the concomitant retrograde metamorphism.     

Symplectites in upper mantle peridotites: Development and implications for the growth of subsolidus garnet,pyroxene and spinel

Silicate-oxide symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa.Harzburgites of olivine and high-Al (1.9–3.6 wt%), Ca (0.6–0.9 wt%) and Cr (0.3–0.9 wt%) enstatite contain symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8–2.2 wt%), Ca (0.1–0.4 wt%) and Cr (0.02–0.8 wt%) enstatite. From textures and mineral chemistries these symplectites are interpreted to have formed by mineral unmixing and migration from Al–Ca–Cr discrete enstatite to adjoining mineral interfaces.Garnet harzburgites are composed of large (0.5–1 cm) olivine, equally large discrete low-Al (0.6–1.1 wt%), Ca (0.1–0.5 wt%), and Cr (0.1–0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel+diopside+garnet, or garnet+spinel. Spinel diopside garnet symplectites have cores of spinel+diopside, resembling symplectites inharzburgites, but surrounded by rims of garnet or garnet+undigested globular spinel. From textures and chemistries we suggest that the spinel+diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside+spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In bothharzburgites andgarnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of symplectite bodies into olivine. Olivine appears to have activated adjacent     

Transmission electron microscopy investigation of the defect microstructure of four natural orthopyroxenes

The defect structure of crustally deformed orthopyroxenes from a dunite, a peridotite, and a pyroxenite are characterized and their defect structures are compared with that of an orthopyroxene of a lherzolite from a volcanic xenolith. The microstructures contained isolated unit dislocations, isolated stacking faults, and Ca-rich, clinopyroxene lamellae. The isolated dislocations have Burgers vectors, b, which were predominantly [001]. The stacking faults have a displacement vector R =1/4[001]. A lamellae consisted of a 1/4 wide Ca-rich region bounded by complex dislocation arrays. These lamellae are usually 100 or more in length and are nearly parallel to the (100) in the matrix. The dislocations in the boundary regions are spaced about 500 Å apart. The lherzolite orthopyroxenes were nearly free of isolated defects, in comparison to the other samples. Annealing at 1390° C for 1 hr produced no detectable recovery of the isolated defects in the orthopyroxene substructure.     

An electron microscopic study of amphibole lamellae in augite

Ion-thinned samples of augite from four plutonic igneous rocks have been examined in the electron microscope at 100 kV. Lamellae less than 0.08 m thick were observed parallel to (010) in all four samples. Electron diffraction shows that the lamellae consist of clino-amphibole, space group I2/m, with crystallographic axes parallel to those of the augite. Analytical electron microscopy of the lamellae in one specimen shows that they are a hornblende. The amphibole lamellae have nucleated at the interface between the augite host and exsolution lamellae of orthopyroxene parallel to (100) of the augite. The interface between the amphibole lamellae and the augite is coherent.It is thought that the amphibole lamellae have formed by exsolution from the augite, implying the existence of finite solid solution between members of the pyroxene and amphibole groups of minerals.     

Exsolution and coarsening in iron-free clinopyroxene during isothermal annealing

Exsolution and coarsening in Fe-free clinopyroxene of composition En_45.1Di_54.9 was studied at temperatures of 1300, 1200, and 1100°C, and annealing times between 10 min and 4320 h. Based on the wavelength of the exsolution lamellae, the exsolution process can be divided into exsolution sensu stricto and coarsening. During exsolution sensu stricto, the average wavelength of the “001” and “100” lamellae remains constant in contrast to the subsequent coarsening process. A progressive development of the microstructure is observed before coarsening which includes formation of island-like regions of dark contrast without phase separation, occurrence of pigeonite and diopside domains predominantly elongated along (100), formation of “100” exsolution lamellae, dissolution of the “100” lamellae, and predominant occurrence of “001” exsolution lamellae. All observations are in accordance with the exsolution mechanism of spinodal decomposition. After termination of the exsolution process, coarsening of the exsolution lamellae is observed. The “001” lamellae coarsen according to the rate law

     

Oriented precipitates of quartz and amphibole in clinopyroxene of metabasites from the Greek Rhodope: a product of open system precipitation during eclogite–granulite–amphibolite transition

In metabasites from the high‐grade metamorphic Kimi Complex of the Greek Rhodope, lamellae of quartz and amphibole were observed together with either rutile or titanite precipitates in the cores of clinopyroxene that underwent a prolonged exhumation history from eclogite via granulite and amphibolite facies conditions. Lamellae are crystallographically oriented along clinopyroxene cleavage planes (010) and interpreted as the result of precipitation during a two‐step intracrystalline compositional adjustment from an omphacite‐ to a Ca‐Tschermak‐rich clinopyroxene and finally to diopside. Even though retrograde diffusion profiles develop in clinopyroxene, the diffusion rates of highly charged ions like Si⁴⁺, Al³⁺and Ti⁴⁺are too slow, so precipitates rich in Si⁴⁺ (quartz), Al³⁺(amphibole) and Ti⁴⁺(rutile or titanite) form as viable kinetic alternatives. These precipitation reactions are not possible isochemically, and additional exchange of ions with the matrix is required for precipitation to occur. This exchange of mainly rapidly diffusing low‐charge ions is postulated to proceed by bulk diffusion (self‐diffusion) and does not require special pathways or fluid, even though these may play an additional role during metamorphism. This hitherto undescribed process of precipitation made possible by diffusional exchange with the matrix through an intact host crystal lattice is termed here as ‘open system precipitation’ (OSP). No prior Ca‐Eskola‐rich (ultrahigh‐pressure, UHP) composition of omphacite is required to explain the observed precipitates. It is proposed that many precipitation phenomena that were hard to explain in a closed‐system scenario can now be explained more readily by OSP. Calculation of an original solid solution composition, as in an UHP phase, from precipitate and host mineral analyses becomes imprecise to the degree that bulk diffusion and hence OSP have been important.     

Coordination of boron in nominally boron-free rock forming silicates: Evidence for incorporation of BO3 groups in clinopyroxene

To explore mechanisms of B-incorporation in common chain silicates we have investigated synthetic diopside samples produced under boron-saturated conditions by ¹¹B and ²⁹Si magic-angle spinning (MAS) NMR and single-crystal NRA, FTIR, EMP and XRD/SREF techniques. Our samples contain 0.14-0.65 wt.% B₂O₃. NMR reveals that B is predominantly present in trigonal coordination in the clinopyroxene structure. This observation is supported by vibrational bands characteristic for B-O stretching in BO₃ groups in the range 1250-1400 cm⁻¹ in polarised single crystal FTIR-spectra. Single crystal structure refinements suggest that boron replaces Si at the T site. Combined, these results suggest that boron replacement for Si at the T-site leads to disruption of one of the T-O bonds of the nominal clinopyroxene structure resulting in replacement of SiO₄ tetrahedra by BO₃ groups. Our results show that high concentrations of boron can be incorporated in the nominally boron-free diopside. Elevated B-concentrations in the present calcic clinopyroxenes are accompanied by modifications of the diopside crystal structure involving the breaking of one T-O bond and simultaneous formation of vacancies at the octahedral M2 site. These structural modifications destabilize the structure and constitute thereby limiting factors for incorporating higher boron concentrations in diopside.     

安徽女山单斜辉石巨晶的地球化学

本文研究了安徽女山14个单斜辉石巨晶。它们属于铝普通辉石、透辉石和铁钠造辉石。主要元素、稀土和微量元素及同位素特征表明：它们是同一碱性玄武岩浆在幔源条件下不同程度结晶分异的产物，相对于寄主岩均属捕虏晶，可能自中生代即已开始形成。     

Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic‐Ultramafic Complexes

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the K?z?lda? ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle-or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the K?z?lda? ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of Ca O(23.6–24.3 wt%), SiO_2(52.2–54.0 wt%), Al_2O_3(0.67–2.15 wt%), Cr_2O_3(0.10–0.42 wt%) and Na_2O(0.14–0.26 wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca~(2+), Fe~(3+), Cr~(3+) and Na+, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe~(3+) and Cr~(3+) preferentially partitioned into magnetite, while Ca~(2+) and Na+ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.     

Equilibrium coexistence of three amphiboles

Electron probe and wet chemical analyses of amphibole pairs from the sillimanite zone of central Massachusetts and adjacent New Hampshire indicated that for a particular metamorphic grade there should be a restricted composition range in which three amphiboles can coexist stably. An unequivocal example of such an equilibrium three amphibole rock has been found in the sillimanite-orthoclase zone. It contains a colorless primitive clinoamphibole, space group P2₁/m, optically and chemically like cummingtonite with blue-green hornblende exsolution lamellae on (100) and (¯101) of the host; blue-green hornblende, space group C2/m, with primitive cummingtonite exsolution lamellae on (100) and (¯101) of the host; and pale pinkish tan anthophyllite, space group Pnma, that is free of visible exsolution lamellae but is a submicroscopic intergrowth of two orthorhombic amphiboles. Mutual contacts and coarse, oriented intergrowths of two and three host amphiboles indicate the three grew as an equilibrium assemblage prior to exsolution. Electron probe analyses at mutual three-amphibole contacts showed little variation in the composition of each amphibole. Analyses believed to represent most closely the primary amphibole compositions gave atomic proportions on the basis of 23 oxygens per formula unit as follows: for primitive cummingtonite (Na_0.02Ca_0.21 ^– Mn_0.06Fe²⁺ _2.28Mg_4.12Al_0.28) (Al_0.17Si_7.83), for hornblende (Na_0.35Ca_1.56Mn_0.02Fe_1.71Mg_2.85Al_0.92) (Al_1.37Si_6.63), and for anthophyllite (Na_0.10Ca_0.06Mn_0.06Fe_2.25Mg_4.11Al_0.47) (Al_0.47Si_7.53). The reflections violating C-symmetry, on X-ray single crystal photographs of the primitive cummingtonite, are weak and diffuse, and suggest a partial inversion from a C-centered to a primitive clinoamphibole. Single crystal photographs of the anthophyllite show split reflections indicating it is an intergrowth of about 80% anthophyllite and about 20% gedrite which differ in their b crystallographic dimensions. Split reflections are characteristic of all analyzed orthorhombic amphiboles so far examined from Massachusetts and New Hampshire except the most aluminous gedrites, and the relative intensity of the gedrite reflections is roughly proportional to the degree of Na and Al substitution. Thin sections of a few of these anthophyllite specimens show lamellae parallel to (010) that are just resolved with a high power objective.Publication approved by the Director, U.S. Geological Survey.     

Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism     

Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation

The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde eclogites, amphibolites and ultramafic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The CCSD-MH consists of four petro-structural units separated by three SE-dipping ductile shear zones DFa (835-1280 m), DFb (2010-2280 m) and DFc (2920-3225 m), which are correspondent with the regional shear zones in the northern Sulu UHP supracrustal zone. Using the electron backscatter diffraction (EBSD) technique, we investigated the lattice-preferred orientations (LPOs) of omphacite, diopside and quartz in core samples from the CCSD-MH. Omphacite from eclogites and diopside from garnet pyroxenites display very strong LPOs, which are characterized by the maximum concentration of [001]-axes parallel to the lineation and (010)-poles normal to the foliation. Quartz in para- and granitic mylonites/gneisses from the shear zones DFa, DFb and DFc developed multiple slip systems. ⁴⁰Ar/³⁹Ar dating of biotite in para- and granitic gneisses from the CCSD-MH yields 223-202 Ma, which constrains the formation ages of the quartz high-temperature prism slip systems {m}<a> and {m}[c]. The asymmetric LPOs of omphacite, diopside, olivine and quartz with respect to the structural frame reveal three deformation phases in the Sulu terrane. In the Middle Triassic, the northward subduction of the Yangtze plate to depths > 100 km produced a top-to-the-south shear sense in LPOs of omphacite, diopside and olivine, and a nearly N-S-striking foliation and a subhorizontal N-S-trending lineation in eclogites and ultramafic rocks. In the Late Triassic, the UHP rocks were exhumed to the lower crust and quartz developed high-temperature slip systems with a top-to-the-NW shear sense, which is consistent with the regional SE-dipping foliation and SE-plunging lineation in the ductile shear zones. In the Cretaceous the UHP rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a NE-striking antiform structure. As a result, the activation of quartz low-temperature basal slip (0001)<a> is characterized by a top-to-the-SE shear sense in the south, but a top-to-the-NW shear sense in the north.