Symplectic Intergrowths Research Articles

Symplectites in High-Grade Pelitic Gneisses of Usilampatti, Tamil Nadu: P-T Conditions and Geochemistry

Abstract Occurrence of fine-grained, fingerprint-like symplectic intergrowths of cordierite, K-feldspar and quartz formed due to the retrogressive, garnet breakdown reactions following decompression during uplift, is reported here from metapelites of Usilampatti area. These metapelites consisting of cordierite, garnet, hypersthene, biotite, hercynitic spinel, sillimanite, K-feldspar assemblage were metamorphosed at 6.5-7.5 Kb and 750-800°C. The symplectites were formed at 600-670°C and 5 Kb pressure. The textural and mineralogical evolutions in these metapelites indicate a rapid decompressional P-T evolution. Petrological and geochemica1 data suggest that these metasediments were derived mostly from an evolved basement source of granodiorite-granite composition with a minor basic component.

Occurrence of Spodumene in the Rare Metal-Bearing Pegmatites of Mariagalla-Allapatna Area, Mandya Dist., Karnataka

Abstract Spodumene bearing rare metal pegmatites are reported mostly from Karnataka in contrast to lepidolite and amblygonite bearing pegmatites described from major mica-fields of India. Tantalite (Ta2O5 50-70%) rich pegmatites intruding the Sargur metasediments of Nagamangala schist belt near Marlagalla- Allapatna are also rich in spodumene (10-15% by vol.) as well as beryl (10.6-11.4% BeO). Light green spodumene (Li2O 6.55-7.35%) occurs randomly as prismatic crystals a few cm to 30 cm in length. Under the microscope, symplectic intergrowth between spodumene and quartz along the contact with albite and replacement of spodumene by quartz and muscovite are noticed. The 'd' spacings and intensities obtained from the X-ray diffraction patterns match well with the published values for Alphaspodumene from Pala San Diego County, California, U.S.A. Some variations observed in the intensities ofinterplanar spacings could be due to preferred orientations exhibited by the mineral. When heated to 1000° C for 3 hours, the Alpha-spodumene transformed to Beta- spodumene. Chemically these spodumenes are "high iron non-chromian" types (FeO+Fe2O3 1.17 to 1.29%, Cr47- 87 ppm). While Mn content is high (1237-1826 ppm), other trace elements such as V, Cu, Co, Pb and Be are less than 10 ppm, Cs 39-98 ppm and Sn 19-28 ppm. Sn values are considerably low as compared to specimens reported from elsewhere, possibly because these pagmatites are devoid of cassiterite. Mineralogical assemeblage ofspodumene-tnntalite-pollucite in these pegmatites places them in "LCT (Lithium-Caesium-Tantalum) family" of rare element pegmatites as per classification of Cerny. Petromineralogical and geochronological studies of Allapatna granites have been undertaken which will throw furthur light on the geochemistry and paragenesis of these pagmatites. Few hundred tonnes of spodumene are expected to be recovered commercially from shallow depths alone along with tantalites and beryl.

Retrograded eclogites in the Olekma granite-greenstone region, Aldan Shield, Siberia

Relict eclogite facies assemblages occur in basic material which forms boudins and isoclinally folded bands in tonalitetrondhjemite gneisses in the northeastern part of the Archaean Olekma granite-greenstone region, Aldan Shield, Siberia. The basic material is interpreted as the remains of dykes which were deformed and metamorphosed during a major orogenic event. The central parts of thicker (3–7 m) basic boudins contain relict garnet and clinopyroxene with symplectic intergrowths of sodic plagioclase. Garnets are surrounded by kelyphitic rims of plagioclase and hornblende, and clinopyroxenes are partially replaced by hornblende. In the margins of the boudins (which have similar bulk composition to the central parts) garnet and clinopyroxene are absent, and the mineral assemblage is hornblende and plagioclase, i.e. an amphibolite facies paragenesis. The petrographic observations combined with mineral chemistry suggest that during the early stages of metamorphism the (anhydrous) basic rocks had garnet +jadeite-rich clinopyroxene assemblages. Subsequently, during decompression and continuing deformation, sodic plagioclase exsolved from the clinopyroxene, and garnet and clinopyroxene were variably recrystallised to give rise to the hornblende + plagioclase amphibolite facies assemblages. The eclogites must have equilibrated at 10–15 kbar, and geothermometry of the amphibolite facies assemblages suggests conditions of 650–700°C and 5.5–7 kbar. During decompression and retrogression of the eclogites, local partial melting of the host gneisses took place, particularly between boudins of mafic material.

Garnet breakdown in some deep seated garnetiferous xenoliths from the central Sierra Nevada: Petrologic and tectonic implications

Garnets in garnetiferous xenoliths from Big Creek, Pick and Shovel and Chinese Peak volcanic pipes, occurring along the axial region of the Sierra Nevada batholith, have broken down to variable degrees to optically opaque and extremely fine-grained kelyphitic rims and seams. In some cases, only opaque pseudomorphs of garnet remain; in others, garnet occurs only as a tiny relict within an opaque mass; in still others, the kelyphite forms a rim and pervades the garnet grain in a complex reticulate fashion. Backscattered-electron imaging and electron microprobe analyses reveal that the unaltered portions of garnets exhibit negligible compositional variations. The unaltered portion is often surrounded by a resorbed rim whose composition is slightly different from that of the bulk garnet; however no systematic difference occurs between the compositions of resorbed rims and those of the unaltered garnets. Around the resorbed rim, lies a zone composed of symplectic intergrowths of aluminous spinel, aluminous orthopyroxene and calcic plagioclase (An 91). Compositions of the symplectite spinels and orthopyroxenes are different from the compositions of those occurring as major discrete phases in the rocks. The possibility that the kelyphites results from oxidation is ruled out. Two alternatives for the origin of these kelyphites seem viable. In the first, they are the result of isochemical decomposition of garnet due to transport by the host lavas from depth to the surface. In the second, they are produced by breakdown and retrogression of garnet due to tectonic transport of the precursor rocks from high P- T conditions to lower P- T conditions before their incorporation into the host volcanics. The second alternative is favored and is supported by the following arguments: (1) Decompression due to transport of the xenoliths by the volcanics should essentially take place along an adiabat and would most likely cause partial melting. No melt phase was found in the symplectites. (2) Such rapid adiabatic decompression would arrest the original high P- T condition of equilibration of the primary xenolith mineralogy. The P- T conditions of equilibration of the primary xenolith mineral assemblages are found to range from the 11–24 kbar and 600–900°C. These low P- T conditions indeed indicate a re-equilibration of the primary mineral assemblages at a shallower depth. (3) The garnet breakdown reaction also involved clinopyroxene and produced an overgrowth of Fe- and Al-rich and Na-poor clinopyroxene around the primary clinopyroxene. (4) Using the known phase compositions, a balanced reaction, which involves primary phases and the symplectite phases, can be written. (5) Thermodynamic modelling of the garnet breakdown phenomenon according to the reaction: CaMgSi 2O 6 + 2Mg 3Al 2Si 3O 12=3Mg 2Si 2O 6 + CaAl 2Si 2O 8 + MgAl 2O 4 yields the P- T condition for garnet breakdown at 841°C-15 kbar for the sample PS-10, for example. The original subsolidus re-equilibration P- T condition of the primary mineral assemblages for this sample is calculated as 892°C-23 kbar. This calculation signifies uplift of the subsolidus garnet clinopyroxenite assemblage along a superadiabatic P- T path. The hypothesis set forth in turn suggests movement of the solid crust-mantle segments along a superadiabatic path from a high P- T regime to lower P- T conditions. Thus the crust-mantle segments underlying the Sierra Nevada batholith, represented by the xenoliths, originated at a greater depth and were tectonically uplifted prior to becoming entrained within the volcanic hosts. This dynamic model is compatible with the geological scenario of the western United States.

Diamondiferous peridotite xenoliths from the Argyle (AK1) lamproite pipe, Western Australia

This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (≈1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al2O3 and CaO < 1%, Na2O≤0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ≠ 91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr2O3 (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ≠ 92–94 ) have comparatively high Cr2O3 (0.2%–0.45%) and Na2O (up to 0.18%) but very low Al2O3 contents (0.5%–0.7%). Diopsides (Mg ≠ 92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr2O3) and have low Al2O3 (0.7%–2.2%) and Na2O (0.5%–1.6%) contents. Many have high K2O contents, typically 0.1%–0.4% but up to 1.3% K2O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe2+)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr2O3, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr2O3) titanate resembling armalcolite but containing significant K2O (1%–2.5%), CaO (0.6%–2.2%), ZrO2 (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly 13C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (≥2 Ga) enrichment in incompatible elements.

Reaction history of garnet‐sapphirine granulites and conditions of Archaean high‐pressure granulite‐facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

ABSTRACT Sequential reaction textures in Archaean garnet‐corundum‐sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse‐grained, high‐pressure (P &gt; 9.5 kbar) granulite‐facies assemblages (M1) to late, low‐pressure (P &lt;6 kbar) granulite‐facies sub‐assemblages (M2).The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late‐M1 boron‐free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine‐grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel).The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time‐sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)‐discontinuous reactions in the FMASH multisystem sapphirine‐garnet‐corundum‐spinel‐cordierite‐gedrite‐kornerupine, the facies‐sequence indicates decompression at essentially constant T assuming constant a(H2O).Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P‐T vectors.Garnet‐biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late‐M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early‐M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high‐pressure granulite‐facies metamorphism was followed by an almost isothermal pressure decrease of &gt; 5 kbar, indicative of rapid uplift. The P‐T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite‐facies terranes.

Contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the Grenville Province of western Labrador

Contrasting coronas around olivines, pyroxenes, and ilmenites occur in metagabbros in two adjacent terranes in the Grenville Province of western Labrador. In the parautochthonous Gagnon terrane, coronas around olivines display evidence of evolution from double coronas of orthopyroxene- and fluorine-rich pargasitic amphibole, through triple coronas of orthopyroxene – garnet (clinopyroxene) – pargasitic amphibole, to double coronas of orthopyroxene – garnet (clinopyroxene) (parentheses enclose phase occurring as inclusions in previous phase). In the allochthonous Lac Joseph terrane, coronas around olivines consist of orthopyroxene – [clinopyroxene–spinel] or orthopyroxene–[pargasitic amphibole – spinel] (brackets enclose phases occurring in symplectic intergrowths). Possible reactions that explain the development of these assemblages are investigated, and it is concluded that the system was closed at the level of the corona. The silica required for the olivine–plagioclase reaction in Gagnon terrane was generated by the FMQ reaction during cooling and controlled the rate of reaction along olivine–plagioclase contacts. Alumina was relatively immobile, resulting in the formation of corundum and spinel inclusions in plagioclases in Gagnon and Lac Joseph terranes, respectively.Evolution of the corona gabbros in Gagnon terrane took place in two stages during isobaric cooling after emplacement, whereas formation of the symplectite coronas around olivines in Lac Joseph terrane is compatible with a single-stage reaction during isobaric cooling following emplacement. It is qualitatively estimated that the garnet-bearing coronas in Gagnon terrane formed at higher pressures than the spinel-bearing coronas in Lac Joseph terrane.Greenschist- and amphibolite-facies assemblages in noncorona gabbros in Gagnon terrane occur in locations corresponding to the regional greenschist- and amphibolite-facies metamorphic zones and are accordingly interpreted as being Grenvillian features. There is no apparent Grenvillian imprint in the gabbros in Lac Joseph terrane.

Petrography and geochemistry of an unusual Fe-rich basaltic komatiite from Boston Township, northeastern Ontario

The Boston Creek Flow is a thick, Archean layered basaltic komatiite lava flow located 16 km south of Kirkland Lake, Ontario, within the Abitibi greenstone belt. It is superficially similar to the layered flows of Munro Township in that it consists of a basal pyroxenite overlain sequentially by peridotite, pyroxenite, gabbro, and pyroxene spinifex-textured rock and exhibits the typical cumulate komatiite crystallization sequence: olivine–clinopyroxene–plagioclase. The spinifex-textured unit is, however, unusually thick (33 m), and in comparison with other Archean lavas of similar magnesium content, the flow is Fe and Ti enriched and Al and Y depleted. CaO/Al2O3 and Al2O3/TiO2 ratios of 2–3 and 5, respectively, also distinguish it from other Archean magnesium-rich volcanic rocks.The preservation of typical tholeiitic chemistry in lavas underlying the Boston Creek Flow precludes metamorphism and metasomatism as explanations of the flow's enigmatic geochemistry. This is supported further by relict igneous textures exhibited by Ti-bearing oxides in the pyroxenite, gabbro, and spinifex-textured units of the flow. The presence of trellis-textured ilmenite lamellae in titanomagnetite, the textural evidence for symplectic intergrowth of titanomagnetite and clinopyroxene during crystallization, and the failure of assimilation models to explain the coupling of Fe and Ti enrichment with Al depletion suggest rather that the unusual geochemistry of the flow is a primary igneous feature. The Boston Creek Flow is more Al depleted and Fe and Ti enriched than Barberton and Minnesota Al-depleted komatiites (ADK) of the same magnesium number. It is therefore an Fe-rich, layered ADK.

Refractory Minerals in Interplanetary Dust

A newly studied interplanetary dust particle contains a unique set of minerals that closely resembles assemblages in the refractory, calcium- and aluminum-rich inclusions in carbonaceous chondrite meteorites. The set of minerals includes diopside, magnesium- aluminum spinel, anorthite, perovskite, and fassaite. Only fassaite has previously been identified in interplanetary dust particles. Diopside and spinel occur in complex symplectic intergrowths that may have formed by a reaction between condensed melilite and the solar nebula gas. The particle represents a new link between interplanetary dust particles and carbonaceous chondrites; however, the compositions of its two most abundant refractory phases, diopside and spinel, differ in detail from corresponding minerals in calcium- and aluminum-rich inclusions.

Replacement of pyroxene by hornblende, isochemically balanced with replacement of plagioclase by garnet, in a metagabbro of upper-amphibolite grade

In a garnet-clinopyroxene-hornblende metagabbro from the Archean basement north of the Cape Smith-Wakeham Bay Belt, Quebec, Canada, the following major mineral reactions have been observed: (1) clinopyroxene → hornblende and (2) plagioclase + hornblende or FeTi oxides → garnet. It is argued that these reactions are synchronous and form part of one solid solution reaction system: H 2O + cpx′ + hbl′ + plag′ + FeTi oxides′ → grt + cpx″ + hbl″ + plag″ + FeTi oxides″ + qtz A new petrological mixing method is described, which, starting from the present reaction products, estimates the proportions and compositions of the reactants for which the reaction system is completely isochemical. The reaction system also explains the observed hornblende-quartz symplectic intergrowths and the abundance of ilmeno-hematite. The isochemical character could also hold for the H 2O involved. The reaction is probably an adjustment to increasing pressure, combined with cooling, and can be correlated with the metamorphic peak of the Hudsonian metamorphic episode.

Fluid inclusions as petrogenetic indicators in granulite xenoliths, Pali-Aike volcanic field, Chile

Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.

Geology and Petrology of Some Polymetamorphosed Amphibolites and Associated Rocks in Northeastern Taiwan

The pre-Tertiary metamorphic complex of northeastern Taiwan consists of schist, marble, gneiss, amphibolite, (meta)granodiorite, and minor serpentinized peridotite. Fault-bounded and foliated amphibolite with the assemblage green hornblende + plagioclase (An 40 to An 52 ) + epidote (Ps 4 to Ps 15 ) + sphene ± rutile ± quartz was intruded by granitic rocks about 87 m.y. ago or earlier. The most apparent thermal effects include (1) transformation of green hornblende to brown hornblende, (2) transformation of epidote to symplectic intergrowths of clinozoisite + plagioclase + quartz, (3) crystallization of clinopyroxene at the expense of hornblende, and (4) production of biotite + muscovite + K-feldspar in the partly; metasomatized amphibolites. The metamorphic-igneous complex was later intruded by thin diabasic dikes; the older of these dikes are believed to be feeders for basaltic flows and pyroclastics in the overlying lower Cenozoic formations. Some dikes may be as young as Miocene. The whole complex—basement, dike, and cover rocks—was metamorphosed under greenschist facies conditions during Pliocene-Pleistocene collision between the China and the Philippine Sea plates. Amphibolite has been partly recrystallized to the assemblage actinolite + chlorite + epidote (Ps 15 to Ps 27 ) + quartz + sphene. Both nonfoliated dike rocks and foliated basaltic flows in the Cenozoic sequence carry the asssemblage albite + actinolite + chlorite + epidote (Ps 20 to Ps 26 ) + biotite + quartz + sphene. Zeolite facies recrystallization occurred locally along fractures in some amphibolite and produced laumontite + epidote (Ps 32 ) + chlorite. Bulk-rock X-ray fluorescence (XRF) compositions of 14 amphibolites fall within the range of low-K and low-Ti tholeiite, whereas 7 diabasic dike rocks and 2 basaltic rocks of the Cenozoic sequence are alkali basalts with relatively higher TiO 2 (0.7 to 2.7 wt %), Na 2 O (1.6 to 4.0 wt %), and K 2 O (0.6 to 0.8 wt %). Thermally recrystallized amphibolites enclosed in the Upper Cretaceous granitic intrusion are enriched in SiO 2 , Al 2 O 3 , and K 2 O and depleted in total Fe, MgO, and CaO. Microprobe analyses of coexisting phases show that (1) zoning of epidote, amphibole, and plagioclase reflects changes in pressure-temperature conditions—with more pronounced fractionations characteristic of lower grades; (2) the Mg/Fe ratio of chlorite is 2.0 in amphibolite, 1.2 to 1.5 in altered amphibolite, and 0.9 to 1.0 in greenschist; and (3) the amphibolite facies hornblendes contain high pargasite-tschermakite proportions (Al 2 O 3 = 8 to 13 wt %; TiO 2 = less than 0.6 wt %; Mg/Mg + Fe ratio = 0.54 to 0.65), the thermally recrystallized brown hornblendes contain substantially more TiO 2 (1.0 to 1.9 wt %), slightly lower Si and hence higher Al IV and elevated total aluminum contents compared to the green and blue-green hornblendes, whereas the greenschist facies amphiboles are actinolitic, with Al 2 O 3 less than 5.9 wt %. Fractionation of elements between coexisting phases in the amphibolites and associated rocks are systematic and, in general, suggest a close approach to chemical equilibrium; in most cases, the exchange reactions appear to be of the ion-for-ion type, except where several structurally distinct crystallographic sites are involved in the partitioning. Comparisons with experimentally determined phase equilibria and element distributions among coexisting minerals yield the following estimates of pressure-temperature conditions for the various recognized stages of metamorphic recrystallization: (1) amphibolite facies metamorphism at pressures of about 5 kb and temperatures approaching 650 °C; (2) accompanying the granitic intrusion, potassium metasomatism and thermal metamorphism at about 700 °C, judging from the incipient breakdown of epidote-clinozoisite in the amphibolite and the formation of clinopyroxene at the expense of hornblende; and (3) Pliocene-Pleistocene greenschist facies metamorphism at 350 to 475 °C and P total of no more than about 5 kb. Cretaceous or earlier sea-floor spreading apparently generated the Suao-Nanao basaltic + ultramafic protolith; these rocks, overlain by younger sediments, were transported to and sequestered at the Asiatic continental margin, accompanied by an intermediate-pressure recrystallization event prior to or during Cretaceous time. Cretaceous calc-alkalic plutonism and thermal metamorphism attended marked convergent plate motion. Only minor (slightly alkalic) mafic magmatic activity was associated with a hypothesized early Cenozoic rifting of the Asiatic continental margin. Greenschist facies recrystallization, which apparently took place during Pliocene-Pleistocene time, is inferred to reflect the collision of the Chinese shelf + slope + rise with the western edge of the Philippine Sea plate (= Luzon arc).

Geology and petrology of some polymetamorphosed amphibolites and associated rocks in northeastern Taiwan: Summary

The pre-Tertiary metamorphic complex of northeastern Taiwan consists of schist, marble, gneiss, amphibolite, (meta)granodiorite, and minor serpentinized peridotite. Fault-bounded and foliated amphibolite with the assemblage green hornblende + plagioclase (An 40 to An 52 ) + epidote (Ps 4 to Ps 15 ) + sphene ± rutile ± quartz was intruded by granitic rocks about 87 m.y. ago or earlier. The most apparent thermal effects include (1) transformation of green hornblende to brown hornblende, (2) transformation of epidote to symplectic intergrowths of clinozoisite + plagioclase + quartz, (3) crystallization of clinopyroxene at the expense of hornblende, and (4) production of biotite + muscovite + K-feldspar in the partly; metasomatized amphibolites. The metamorphic-igneous complex was later intruded by thin diabasic dikes; the older of these dikes are believed to be feeders for basaltic flows and pyroclastics in the overlying lower Cenozoic formations. Some dikes may be as young as Miocene. The whole complex—basement, dike, and cover rocks—was metamorphosed under greenschist facies conditions during Pliocene-Pleistocene collision between the China and the Philippine Sea plates. Amphibolite has been partly recrystallized to the assemblage actinolite + chlorite + epidote (Ps 15 to Ps 27 ) + quartz + sphene. Both nonfoliated dike rocks and foliated basaltic flows in the Cenozoic sequence carry the asssemblage albite + actinolite + chlorite + epidote (Ps 20 to Ps 26 ) + biotite + quartz + sphene. Zeolite facies recrystallization occurred locally along fractures in some amphibolite and produced laumontite + epidote (Ps 32 ) + chlorite. Bulk-rock X-ray fluorescence (XRF) compositions of 14 amphibolites fall within the range of low-K and low-Ti tholeiite, whereas 7 diabasic dike rocks and 2 basaltic rocks of the Cenozoic sequence are alkali basalts with relatively higher TiO 2 (0.7 to 2.7 wt %), Na 2 O (1.6 to 4.0 wt %), and K 2 O (0.6 to 0.8 wt %). Thermally recrystallized amphibolites enclosed in the Upper Cretaceous granitic intrusion are enriched in SiO 2 , Al 2 O 3 , and K 2 O and depleted in total Fe, MgO, and CaO. Microprobe analyses of coexisting phases show that (1) zoning of epidote, amphibole, and plagioclase reflects changes in pressure-temperature conditions—with more pronounced fractionations characteristic of lower grades; (2) the Mg/Fe ratio of chlorite is 2.0 in amphibolite, 1.2 to 1.5 in altered amphibolite, and 0.9 to 1.0 in greenschist; and (3) the amphibolite facies hornblendes contain high pargasite-tschermakite proportions (Al 2 O 3 = 8 to 13 wt %; TiO 2 = less than 0.6 wt %; Mg/Mg + Fe ratio = 0.54 to 0.65), the thermally recrystallized brown hornblendes contain substantially more TiO 2 (1.0 to 1.9 wt %), slightly lower Si and hence higher Al IV and elevated total aluminum contents compared to the green and blue-green hornblendes, whereas the greenschist facies amphiboles are actinolitic, with Al 2 O 3 less than 5.9 wt %. Fractionation of elements between coexisting phases in the amphibolites and associated rocks are systematic and, in general, suggest a close approach to chemical equilibrium; in most cases, the exchange reactions appear to be of the ion-for-ion type, except where several structurally distinct crystallographic sites are involved in the partitioning. Comparisons with experimentally determined phase equilibria and element distributions among coexisting minerals yield the following estimates of pressure-temperature conditions for the various recognized stages of metamorphic recrystallization: (1) amphibolite facies metamorphism at pressures of about 5 kb and temperatures approaching 650 °C; (2) accompanying the granitic intrusion, potassium metasomatism and thermal metamorphism at about 700 °C, judging from the incipient breakdown of epidote-clinozoisite in the amphibolite and the formation of clinopyroxene at the expense of hornblende; and (3) Pliocene-Pleistocene greenschist facies metamorphism at 350 to 475 °C and P total of no more than about 5 kb. Cretaceous or earlier sea-floor spreading apparently generated the Suao-Nanao basaltic + ultramafic protolith; these rocks, overlain by younger sediments, were transported to and sequestered at the Asiatic continental margin, accompanied by an intermediate-pressure recrystallization event prior to or during Cretaceous time. Cretaceous calc-alkalic plutonism and thermal metamorphism attended marked convergent plate motion. Only minor (slightly alkalic) mafic magmatic activity was associated with a hypothesized early Cenozoic rifting of the Asiatic continental margin. Greenschist facies recrystallization, which apparently took place during Pliocene-Pleistocene time, is inferred to reflect the collision of the Chinese shelf + slope + rise with the western edge of the Philippine Sea plate (= Luzon arc).

SEM/microprobe study of some symplectic intergrowths replacing cordierite

Scanning electron microscopy (SEM), involving images produced by back-scattered electrons, reveals the microstructures of symplectic intergrowths replacing cordierite, which generally are too fine-grained and/or intricately intergrown for investigation using an optical microscope. Microprobe analysis, combined with the SEM technique, enables accurate identification of the intergrown minerals, and permits inference of equations for the local chemical changes involved in the formation of the intergrowths. Generally, the local replacement appears to have involved partial reactions taking place in a larger reactive system. Intergrowths forming partial pseudomorphs of cordierite in three metamorphic complexes consists of up to three minerals, only two of which comprise the intergrowth in any one domain. The following intergrown pairs of minerals have been identified: quartz-Al 2SiO 3, quartz-biotite, biotite-oligoclase, biotite-K-feldspar, muscovite-oligoclase, oligoclase-andalusite, K-feldspar-andalusite, aluminous orthopyroxene-Al 2SiO 3, and aluminous anthophylite-Al 2SiO 3.

Pseudomorphous replacement of cordierite by symplectic intergrowths of andalusite, biotite and quartz

High-grade metapelitic gneisses rich in cordierite and K-feldspar at Cooma, N.S.W., Australia, show microstructural evidence of partial to complete replacement of cordierite grains with biotite inclusions by fine-grained, symplectic intergrowths of andalusite, biotite and quartz. The replacement occurred during the later stages of the history of the Cooma Complex, under retrograde conditions. An equation for the local reaction: cordierite+red-brown biotite+H 2O+K + ⇆ andalusite+green-brown biotite+quartz+H + can be balanced approximately, using compositions obtained from microprobe analyses of the minerals involved. The volumes occupied by the solid reactants and solid products are nearly identical. The potassium and water were derived from an unknown external source, possibly connected with the replacement of K-feldspar by muscovite in nearby rocks.

Pallasite meteorites—mineralogy, petrology and geochemistry

Pallasites are highly differentiated meteorites and provide a unique sample from the deep interiors of solar system parent bodies. They contain evidence of the former existence of one or more residual melts. Olivine is a major phase. Its primary shape is rounded; the angular crystals in many pallasites are secondary. Tubular inclusions are widespread. They perhaps are the residence of CO 2, released during laboratory heating experiments. Phosphoran olivine, a new variety of olivine containing 4–5 wt% P 2O 5, occurs in a few pallasites. Its Fe/Mg ratio is apparently independent of the host olivine composition. Pyroxene (not previously described from pallasites) occurs in symplectic intergrowths in seven meteorites. Compositionally, it lies in the gap between pyroxenes in chondrites and most irons. There are two groups: Fs 11.6 ± 0.2 and Fs 16.7 ± 0.2 The pyroxene contains exceptionally low Ca (< 0.1–0.2 wt%) and there is an indication of an inverse relation between Fe and Ca. Modal analyses and density measurements were made on all available specimens and bulk compositions were calculated. The ‘average’ pallasite contains 65 vol. % olivine and 50.5 wt % total Fe. Many of the densities of pallasites cluster around that calculated for close-packed olivine. Pallasites are exotic cumulates. Their textures resemble terrestrial cumulates, as does the presence of olivine and chromite. The metal texture resembles a solidified intercumulus liquid. Those pallasites containing olivine in excess of close-packing were subjected to adcumulus growth, thereby also explaining the widespread mutual borders. There is abundant evidence of deformation. For olivines this includes their fragmental shape and kink banding. Troilite formed a eutectic-like melt with kamacite: pieces of spalled olivine and schreibersite were injected into and captured by this melt. Troilite polycrystallinity resulted from the deformation. This deformation occurred while the pallasites were still deeply buried, resulting in incipient spheroidization of olivine fragments, including the formation of elongate, rounded crystals. A later, lower temperature deformation disrupted plessite. Pallasites formed in multiple parent bodies by processes that recurred in several places within the solar system, as shown by the mineralogical and textural similarities between pallasites that differ in their isotopic and trace element compositions. Type IIIB irons still seem the most likely associated meteorites. Two new pallasites, Dora and Rawlinna, are described briefly.

Petrology and emplacement of the Los Pinos pluton, southern California

The Los Pinos gabbro is located on the eastern margin of the gabbro sub-belt of the Peninsular Ranges batholilh. Two lithologic sequences are recognized in this pluton. The outer group consists of bimodal assemblages of hornblende and plagioclase in several textural varieties, whereas the inner group contains a variety of olivine–plagioclase rocks. Symplectitic intergrowths of spinel, amphibole, and orthopyroxene have developed wherever the olivine and plagioclase were in contact. The outer group is generally the older of the two, although there is evidence suggesting that their emplacement overlapped in time. The contact between the two groups is characterized locally by sequences of comb layers up to 65 m thick. Major-element chemical analyses emphasize the differences between the two groups. The outer group is generally richer in total alkalis, titanium, and silica. Variation diagrams suggest that different differentiation mechanisms operated within each group.Textural and published experimental data suggest that the two groups are derived from a parent magma by clinopyroxene fractionation at PT &gt; 5 kbar. The more silicic differentiates were emplaced at shallower levels into a series of regionally metamorphosed pelitic rocks. Volatile streaming in restricted channels along a conduit zone and periodic degassing of the melt, possibly through volcanic eruption, produced the comb layering in the summit region. The remaining fraction intruded along the old conduit zone. Differentiation at these shallower levels (PT &lt; 5 kbar) produced the olivine–plagioclase cumulates. The symplectic intergrowths in these units formed at this shallower level prior to the solidification of the melt.

A lunar rock of deep crustal origin: sample 76535

Lunar sample 76535 is a coarse-grained troctolitic granulite exhibiting a texture indicative of long annealing times. It is composed of homogeneous crystals of plagioclase (58 per cent, An 96), olivine (37 per cent, Fo 88) and bronzite (4 per cent, En 86). Chromian spinel-bronzite-diopside (Wo 46En 50Fs 4) symplectic intergrowths commonly occur along olivine-plagioclase boundaries and as tiny inclusions within olivine grains. These symplectites apparently formed by a reaction of the type: OI + An + Chromite → Opx + Cpx + Al-Mg-chromite . The reaction is related to the experimentally determined reaction OI + An = Opx + Cpx + Sp of Kushiro and Yoder (1966). The enstatite content of the diopside coexisting with the bronzite indicates equilibration at about 1000°C. Thermodynamic calculations for 1000°C indicate that the symplectites formed at a minimum pressure of about 0.6 kb. Low alumina contents of the pyroxenes indicate equilibration near this minimum pressure. Clusters of the same assemblage found in the symplectic intergrowths, but containing accessory metal, troilite, Ca-phosphates, baddeleyite, plagioclase and/or K-feldspar occur sporadically throughout the rock. These apparent late stage products crystallized in the low temperature-high pressure region discussed above. Phase relations of co-existing metal phases indicate that the rock cooled at a few tens of degrees/my, corresponding to depths of 10–20 km below the lunar surface, in agreement with the above pressure estimate. We infer that 76535 represents an original cumulate deposited at a depth between about 10 and 30 km. The last liquid crystallized in the relatively high pressure-low temperature field opx + cpx + Al- Mg- chromite. Cooling was extremely slow and accompanied by extensive chemical and textural re-equilibration. Reaction to form the symplectites occurred during the late stages of re-equilibration.

Chrome-spinels from the Massif du Sud, southern New Caledonia

SummaryApproximately forty, variously shaped, chromitite bodies occur in dunites and harzburgites of the alpine-type peridotite complex of southern New Caledonia. The chromitites consist of pichrochromite (59 to 42% Cr2O3) and serpentinized olivine (Fo96-92) and display a variety of textures (massive, disseminated, and orbicular) consistent with a magmatic cumulate origin. Dunites contain olivine (Fo93−97) and 1 to 3% picrochromite (50 to 39% Cr2O3). Harzburgites have less than 1% spinel, ranging from picrochromite to ceylonite (45 to 28% Cr2O3) and often forming symplectic intergrowths with bronzite. Temperatures of crystallization of the cumulate picrochromites are believed to have been about 1200 °C while the more aluminous symplectic spinels equilibrated under different conditions and may reflect a late stage recrystallization episode.