The petrology of some volcanic rocks from the British Tertiary province: The islands of Rhum,Eigg, Canna,and Muck

On Rhum, Eigg, Canna and Muck Tertiary volcanics rest upon a Mesozoic or Pre-Mesozoic basement. Aphyric, olivine-phyric, and plagioclase-phyric basalts are recognized. The aphyric basalts are mildly alkaline or transitional types with either a few percent normative nepheline or normative hypersthene. They have anomalously low concentrations of Rb, Sr and K₂O compared to Tertiary tholeiites from the same province.Aphyric hawaiites, and mugearties are found on all the islands, but are particularly abundant on Rhum. The volcanics from Bloodstone Hill, Rhum, originally described as mugearites are anomalous in that they are quartz normative and contain both augite and hypersthene, in contrast to the normal one-pyroxene rocks of the alkali basalt-trachyte association (Muir and Tilley, 1961). These volcanics have closer affinities to the icelandites, the presence of basic plagioclase xenocrysts suggesting an hybrid origin.Olivine and plagioclase are involved in the low-pressure fractionation of the transitional basalts, whereas pyroxene and titanomagnetie play only minor roles. Consequently, the suppression of titanomagnetite crystallization results in an initial trend towards iron enrichment. The presence of both oversaturated and undersaturated derivitives following the hawaiite stage of differentiation, reflects variation in the amount of extracted pyroxene and titanomagnetite.Felsites and pitchstones intrude the volcanic pile on Eigg. The felsites carry corroded quartz crystals and rare alkali feldspar. The more crystal rich pitchstones generally contain augite, hypersthene, zoned plagioclase and titanomagnetite. One from Rudh an Tancaird contains alkali feldspar, titanomagnetite and ferrohedenbergite.Whole rock analyses and microprobe analyses of feldspars and pyroxenes indicate that the acid volcanics are not genetically related to the basalt-hawaiite-mugearite lineage. The felsites appear to have been derived from Torridonian arkose by partial melting, but the pitchstones could only be derived by anatexis of Lewisian gneiss basement (see Dunham, 1968) substantially more basic than that outcropping on Rhum.It is suggested that the low concentrations of Rb, Sr, and K₂O in the alkaline and transitional basalts, mitigates against extensive pre-eruptive differentiation. Possibly the basalts could have been derived by partial melting of a mantle depleted in these elements.     

Distribution of elements in coexisting minerals and the problem of chemical disequilibrium in metamorphosed basic rocks

Distribution of elements in coexisting minerals—biotite, hornblende, augite, hypersthene and plagioclase in charnockitic rocks of West Uusimaa Complex, Finland, is mostly orderly indicating a close approach to chemical equilibrium. The distribution of iron and magnesium in coexisting hornblende and pyroxenes of basic charnockites and other rocks of granulite facies from several different areas is also orderly but the variation in the fugacities of H₂O and H₂ may cause a disorderly distribution locally in some rocks. The probable oxidation or reduction reactions are discussed on the basis of thermochemical and mineralchemical data.     

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: multisaturated liquids and coexisting augites

Basaltic liquids, saturated at one-atmosphere in augite, olivine, and plagioclase, show a strong linear trend in pseudo-quaternary projections from nepheline to slightly quartz normative compositions. Coexisting augites show a corresponding variation from slightly nepheline to hypersthene normative compositions. Consequently, alkalic and mildly alkalic liquids coexist with nepheline normative augites; tholeiitic liquids coexist with hypersthene normative augites; and transitional liquids coexist with augites compositionally intermediate between those crystallizing in alkalic and tholeciitic melts. The variation in augite composition is due to a decrease in wollastonite and non-quadrilateral components, going from mildly alkalic, to transitional, and tholeiitic augites. The observed liquid and augite trends lack correlations between normative composition and crstallization temperature. This is an artifact of plotting unrelated liquid lines of descent. Individual liquid lines of descent, in contrast, display good correlation between normative composition and crystllization temperature, which, for both liquids and coexisting augites, are highly oblique to nearly perpendicular to the general trend. Because the composition of the first augite to appear on the liquidus varies as a function of the bulk system, the location of the thermal divide shows corresponding variations. As a consequence, unifying cotectics and pseudo-quaternary normative projections should not, without specific experimental knowledge, be used to address liquid lines of descent and common magmatic parentages.     

Compositional relations of coexisting orthopyroxene,pigeonite and augite in a tholeiitic andesite from Hakone Volcano

The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO₃ between groundmass pigeonite and augite, compared with 35 per cent CaSiO₃ between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO₃-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.     

Late Ordovician island‐arc volcanic rocks,northern Capertee Zone,Lachlan Fold Belt,New South Wales

The Cape Hoskins volcanoes form part of the Quaternary volcanic island arc that extends from Rabaul in the east to the Schouten Islands in the west, and they overlie the northerly dipping New Britain Benioff Zone. The products of the volcanoes range in composition from basalt to rhyolite, and are normative in quartz and hypersthene. They contain phenocrysts of plagioclase and subordinate augite, hypersthene, and in most samples iron‐titanium oxides; some samples also contain olivine or quartz or both, and some pumice contains hornblende and, rarely, biotite.

Chemical analyses of 29 volcanic rocks are presented; 22 were also analysed for 17 minor elements — Rb, Ba, Sr, Pb, Zn, Cu, Zr, Y, Ni, Co, Sc, Cr, V, Ga, B, U, and Th.

Chemically the rocks have many of the characteristics of the ‘island arc tholeiitic series’, but do not show a pronounced relative enrichment in iron and appear to be relatively enriched in Sr. Compared with volcanic rocks from the northern part of the Willaumez Peninsula, they are lower in K (but not Na), Ti, Rb, Ba, Zr, Pb, Th, Ni, and probably also V, Cu, and Zn: these differences are attributed to the greater depth of the Benioff Zone beneath the Willaumez Peninsula. The more basic of the Cape Hoskins rocks are similar in most respects to lavas of comparable composition from Ulawun volcano to the east.     

Petrology and Ar/Ar chronology of Erdembaba and Kuyucak volcanics exposed along the North Anatolian Fault Zone (Eastern Pontides,NE Turkey): Implications for the late Cenozoic geodynamic evolution of Eastern Mediterranean region

The Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt (EPOB), which geographically corresponds to the northeastern part of Turkey, is still controversial due to lack of systematic geological, geochemical and chronological data. This paper provides new geochemical and chronological data from the Kuyucak and Erdembaba volcanics exposed along the North Anatolian Fault Zone, which is one of the most seismically-active intracontinental strike-slip fault systems on the globe, in the Reºadiye (Tokat) area that is situated in the southern part of the EPOB. Kuyucak volcanics consist predominantly of basaltic rocks including mainly olivine, augite and plagioclase. Erdembaba volcanics comprise mainly dacite and minor trachydacite-trachyandesite that are composed mainly of augite, plagioclase, hornblende and rarely biotite. The Geochemical properties of the Erdembaba volcanic define small amounts of magma mixing events and it follow a linear trend from a primitive end member through to upper crustal evolution. The ⁴⁰Ar/³⁹Ar age determinations of whole rock of volcanic rock samples yielded the ages ranging from 1.33 to 6.31 Ma (late Miocene- Pleistocene). Both groups display an enrichment in LIL and LREE elements relative to HFS and HREE elements, strong negative Nb and Ti anomalies. All geochemical and Ar/Ar chronological data indicate that the late Miocene Kuyucak volcanic rocks, which are characterized by low concentrations of SiO₂ (<50 wt.%), were generated by mantlederived magmas that underwent contamination processes during their transfer to the surface. Whereas, Erdembaba volcanic rocks including high concentrations of SiO₂ were derived from partial melting of the plunging continental crust beneath the Eurasian plate in Pliocene, after closure of Neotethys ocean that was situated in the southern part of the EPOB.     

Petrography and Geochemistry of St. Mary Islands,near Malpe,Dakshina Kannada District,Karnataka

The present paper documents the petrography, mineral chemistry and petrochemistry of the acid volcanics of St.Mary group of islands. These are essentially characterized by pyroxene bearing rhyolites with average 73.49 wt.% SiO₂.All the rhyolites from these islands are acmite normative, hence peralkaline with phenocrysts of zoned plagioclase and sometimes hypersthene/augite. There is depletion of LREE with moderate negative Eu anomaly. The enhancement of HFSE elements indicates their compatibility with continental crust. These rhyolites have high equilibrium temperature (915°C) at relatively lower oxygen fugacity (10⁻¹⁶). The acidic lava flows are anorogenic, within plate rhyolites, generated during northern migration of Indian plate over Marion hotspot.     

藏北可可西里中新世钾质火山岩矿物化学及温压计算

对藏北可可西里中新世钾质火山岩的主要斑晶矿物化学成份进行了电子探针分析,结果表明,火山岩中的辉石为普通辉石、紫苏辉石和古铜辉石,角闪石为钙质角闪石,云母为钛铁黑云母和富铁黑云母,长石为中长石、更长石、钠透长石和透长石,不透明矿物主要为钛铁矿和磁铁矿,可见钛铁矿?磁铁矿固溶体。根据火山岩中单斜辉石?熔体温压计估算的辉石斑晶的形成温度和压力分别为1 065～1 100 ℃和5.3～9.1 kbar。二辉石地质温度计结果表明,辉石的共结温度为976～1 020 ℃。而钛铁矿?磁铁矿固溶体平衡计算结果表明,钛?磁铁矿的平衡温度为841～974 ℃,logfO2为－13.71～－10.87。研究结果显示,可可西里火山岩岩浆房的埋深深度小于30 km,中?酸性系列火山岩是岩浆房在相对低压条件下的中上部地壳部位,经分离结晶后形成。     

Coexisting Ca-poor pyroxenes in the Main Zone of the Bushveld Complex

Electron microprobe analyses of Ca-poor pyroxenes in gabbroic rocks of the Main Zone of the Bushveld Complex reveal that inverted pigeonites have lower Mg/Fe ratios than coexisting hypersthenes. Textural relationships, however, indicate that the two Ca-poor pyroxenes did not crystallize simultaneously from the magma. Early pigeonite reacted with the magma to form hypersthene and the difference in the Mg/Fe ratio of these two pyroxenes reflects the difference of this ratio between early pigeonite and the magma at the time of reaction. Some of the grains of early pigeonite, now inverted to hypersthene, evidently escaped this reaction with the magma. Bulk compositions of pyroxenes intermediate between that of pigeonite and hypersthene are postulated on the grounds of varying amounts of exsolved augite in the hypersthene which has originated from pigeonite by reaction with magma.     

ON THE PHENOCRYSTIC PIGEONITE IN AN ANDESITE FROM MONIWA SENDAI CITY

Phenocrysts of pigeonite are found in an augite-hypersthene-pigeonite andesite in the quarry of Moniwa, in the southwest of Sendai City. Three types of occurrence of pigeonite are noticed in this rock; i.e., single crystal, intergrowth with augite, and inverted crystal from hypersthene. The optical properties of pigeonites of all these types are constant; 2 V = 0°, c∧Z = 41°, α = β = 1. 694 (average). The inversion from hypersthene to pigeonite occurred at En:Fs ratio of about 65:35.

From the modes of occurrence it is concluded that the pigeonite was crystallized alone in the latest intratelluric stage, and that both pigeonite and augite were crystallized simultaneously in the groundmass stage. Occurrence of the augite in the effusive stage is a remarkable fact, which has never been found in the phenocrystic pigeonite-bearing rocks. Further study is expected to throw light on its genetic significance.—author's English summary.     

Geochemical Evolution and Unusual Pyroxene Chemistry of the MD Tholeiite Dyke Swarm from the Archaean Craton of Southern West Greenland

The MD dyke swarm is composed of four generations of large basictholeiite dykes which cut the entire Archaean craton of southernWest Greenland. The four successive generations (MD1, MD2, MD3a,MD3b) are characterized by their orientation and cross-cuttingrelationships and by their mineralogy, texture and progressivelyevolved tholeiitic chemistry. Rare-earth element (REE) abundancessuggest that the dykes may have a fairly complex petrogeneticevolution. The suite varies from early (MD1) heteradcumulatenorites to ophitic and sub-ophitic gabbroic and doleritic rocks(MD2 and MD3) and the youngest generation (MD3b) comprises plagioclase-phyricdolerites. The pyroxene chemistry parallels the geochemical evolution ofthe dykes showing an overall Fe-enrichment trend. However, theclinopyroxenes are enigmatic in that, although they occur predominantlyas part of medium and coarse-grained holocrystalline textures,they are chemically highly variable and calcium-poor, many plottingin the metastable field in the system MgSiO₃ (En)-CaSiO₃ (Wo)-FeSiO₃(Fs). Many individual grains are extremely complex and may beregularly or irregularly zoned. Along with more typical pyroxene forms, the MD1 dykes containpyroxene dendrites poikilitically enclosed by plagioclase. Thedendrites vary compositionally from hypersthene bases to branchesof pigeonite and subcalcic augite and terminate in augite branchtips. The MD2 and MD3a dyke pyroxenes are the most complex.The majority of them are sub-ophitic grains, many with successivezones of orthopyroxene, pigeonite, subcalcic augite, augiteand ferroaugite. However, Ca-enrichment or Ca-depletion, Fe-enrichmentor Fe-depletion and apparently opposing zoning trends can occurin neighbouring grains. Even small interstitial pyroxenes showa very wide range of compositions. Morphologically unusual andcomplex clinopyroxene ‘cylinders’ occur in someof the MD3a dykes. They are chemically relatively uniform andare normal tholeiitic augites. The MD3b rocks have small concentricallyzoned sub-ophitic pyroxenes which show Ca-enrichment with arelatively constant Fs component (29 to 39 mol. per cent). Themost extremely zoned grains have hypersthene cores with successivecoronas of pigeonite and subcalcic augite and have margins ofaugite or ferroaugite. The present ‘coexistence’of such compositionally widely variable pyroxenes and the extremeand often irregular nature of their chemical zoning make thedetermination of true original coexisting pyroxene phases andthe use of a two pyroxene geothermometer very difficult andof limited significance. The presence of a wide variety of pyroxenes of apparently bothstable and metastable compositions in these holocrystallinedykes suggests that these rocks have undergone a complex andrather unusual cooling history. The principal genetic factorswhich could have influenced their crystallization are (1) supercooling,(2) the evolution of discrete interstitial liquid cells, (3)augite-pigeonite peritectic reactions and (4) plagioclase growthand delay of pyroxene nucleation during supercooling of liquidto below the basalt liquidus.     

Partition coefficients for trace elements in silicic magmas

Trace element partition coefficients for 29 elements, including the rare earths, have been calculated for augite, hypersthene, sanidine, plagioclase, quartz, biotite, titanomagnetite and ilmenite from rhyodacite and rhyolite at Twin Peaks, Utah. Partition coefficients are intermediate to those reported from dacite and high silica rhyolite. At Twin Peaks, as in less silicic magmas, partitioning between phenocrysts and melt is governed primarily by crystal structure constraints as opposed to very high silica systems where the structure and volatile content of the melt become the dominant control of trace element partitioning.     

High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Experimental results

Melting experiments have been performed on a primitive, mildly alkalic glassy lava (10 wt.% MgO) from the 1965 eruption of the Surtsey volcano located at the tip of the south-eastern propagating rift zone of Iceland. At atmospheric pressure, approximately on the FMQ oxygen buffer, olivine (Fo₈₁) crystallizes from 1240°C, followed by plagioclase (An₇₀) from 1180°C and augite from 1140°C. The experimental glasses coexisting with olivine, plagioclase and augite are ferrobasaltic enriched in FeO (13.6–14.2 wt.%) and TiO₂ (4.0–4.4 wt.%). In high pressure, piston-cylinder, graphite-controlled runs, olivine occurs as the liquidus phase until 14 kbar, above which augite is the liquidus phase. Low-Ca pyroxene is not a liquidus phase at any pressure. The high pressure liquids are, relative to the one atmosphere liquids, significantly enriched in Al₂O₃ and Na₂O and depleted in CaO as a result of changes in the crystallizing assemblages. Furthermore, liquidus augite is dominantly subcalcic and shows significant enrichment in Al and depletion in Ti. Subliquidus plagioclase is enriched in sodium relative to low pressure phase compositions. Evaluated in normative projections, contrasting liquid lines of descent are revealed as a function of pressure. At one atmosphere, the multisaturated liquids are located close to the thermal divide defined by the plane olivine-plagioclase-augite, but appear, with advanced degrees of crystallization, to be moving away from the thermal divide toward normative quartz. The augites crystallizing in the one atmosphere experiments are calcic and slightly nepheline normative. In the 10 and 12.5 kbar experiments, the augites become subcalcic and dominantly hypersthene normative. Because of this shift in augite compositions, transitional basaltic liquids may at high pressure evolve from the tholeiitic side of the olivine-plagioclase-diopside normative divide onto the alkalic side. With increasing pressure above 15 kbar, the liquidus augite compositions move back toward the olivine-plagioclase-diopside normative divide.     

抚顺—清原地区鞍山群块状硫化物矿床的区域地质特征

抚顺-清原地区位于辽宁省东北部,面积约2500Km~2.区内太古代变质岩系广泛发育,其下部为深变质的麻粒岩相地层,上部为中深变质的角闪岩相地层,后者产出红透山式铜锌矿床多处(属红透山地区),以及张胡子沟铜矿床和大荒沟硫铁矿床(属清原北部地区)等一系列块状硫化物矿床(图1).矿石中铅同位素年令为25～26亿年,在形成时代上可以和鞍本地区鞍山群对比.     

Volcanism on a Proterozoic continental margin in northwestern Queensland

The oldest rocks exposed in northwestern Queensland are metamorphosed calc-alkaline volcanics (Leichhardt Metamorphics), which are intruded by elongate tonalitic to granitic batholiths (Kalkadoon Granite). These rocks are overlain by a less metamorphosed sequence containing basic lavas (Magna Lynn Metabasalt) overlain by extensive ignimbritic rhyolite and dacite (Argylla Formation). Sequences of basalt and psammite overlie the rhyolite unconformably and are overlain in turn unconformably by psammitic, pelitic, carbonate and possibly evaporitic sediments and minor volcanics. Younger granites intrude these rocks.The mineral assemblages of the Leichhardt Metamorphics, Magna Lynn Metabasalt and Argylla Formation indicate greenschist and lower amphibolite facies of metamorphism. The rocks contain no glass and some are obviously recyrstallized; however, phenocrysts, lithic fragments, spherulites, amygdales and flow-top breccias are still recognizable.Sixty-nine of the least deformed volcanic and sub-volcanic rocks were analysed for major elements and up to twenty trace elements. Element dispersion in these analyses indicated that metasomatism was probably of limited extent. The Magna Lynn Metabasalt is similar to low-potassium tholeiite, as it has less than 0.5% potash, high normative hypersthene, some normative quartz and typical Ti/Zr/Y ratios. The acid volcanics have calc-alkaline affinities although andesite is not common and the alumina content is relatively low. They have high K/Na ratios and their trace elements (especially Ba, Sr, Rb, Zr and Ce) are similar to Andean volcanics. The tholeiitic sequences that overlie the calc-alkaline volcanics in northwestern Queensland resemble the basaltic sequences of western U.S.A. that also overlie calc-alkaline volcanics.The predominantly calc-alkaline volcanics of northwestern Queensland are believed to have formed at a continental margin similar to that in the Andean region. The younger tholeiitic lavas and minor continental acid volcanism possibly accompanied crustal tension. Later regional metamorphism and intrusion of large granite batholiths stabilized the region.     

Thermal Metamorphism in the trial harbour district,Tasmania

A Devonian granite complex intrudes Precambrian and Silurian siltstones and sandstones as well as (?) Cambrian volcanics and dunite.

Metamorphism of the Precambrian sediments is slight, and an andalusite‐bearing, pelitic hornfels is the only characteristic assemblage. The (?) Cambrian volcanics give rise to a variety of assemblages; (1) lime‐ and ferromagnesia‐rich (hypersthene — cummingtonite — labradorite; diopside — hornblende — labradorite); (2) magnesia‐rich (cordierite — hypersthene; cordierite — anthophyllite); (3) ultrabasic (olivine and/or pleonaste). Biotite (or phlogopite) is an almost invariable component, and garnet may also be present in these groups.

No significant metamorphism of the dunite is evident; minor development of veins and segregrations of aragonite, magnetite, phlogopite, brucite, chalcedony and antigorite may result from low‐grade hydrothermal activity Metamorphic assemblages in calcareous Silurian siltstones contain garnet, diopside, calcite and epidote.

A characteristic feature of the contact metamorphic aureole is the occurrence of diopside‐rich bodies in granite, volcanic hornfels, quartzite and dunite host rocks.     

Luna 20 pyroxenes: exsolution and phase transformation as indicators of petrologic history

Pyroxenes from our sample of Luna 20 soil are predominantly orthopyroxene with subordinate pigeonite. The orthopyroxenes are chromium-rich bronzites and contain submicroscopic lamellae of augite in a twinned orientation exsolved on (100). These lamellae have a composition close to the diopside-hedenbergite join. Asymmetric diffuse streaks parallel to $\text{a}{}^1$ indicate stacking faults parallel to (100) and possibly very thin (10–20 Å) lamellae of clinobronzite parallel to (100). Pigeonite crystals are very complex crystallographically and chemically, with optically visible (001) augite exsolution lamellae and two sets of chromite exsolution lamellae. In addition, there are submicroscopic (100) augite lamellae and a second generation of clinohypersthene lamellae which appear to have exsolved from the (001) augite lamellae. The clinohypersthene host, which has a large number of stacking faults parallel to (100), has partially inverted to hypersthene of the same composition. The hypersthene occurs as very fine lamellae (less than 1000 Å) parallel to the (100) plane of the clinohypersthene. X_D^Fe-Mg values for five host-lamellae pairs in pigeonite K-4 indicate a significant amount of subsolidus readjustment. We tentatively conclude that many of the bronzite and pigeonite crystals were derived from rocks crystallized from a high level magma chamber in the lunar highland crust.     

藏南定结淡色花岗岩--基底隆升降压熔融成因的地质证据

西藏南部定结地区高喜马拉雅结晶基底中淡色花岗岩体紧靠藏南拆离断层内部产出．野外地质和岩相学特征显示其为S型、分两期侵入的淡色花岗岩体——早期的黑云母淡色花岗岩和晚期的白云母淡色花岗岩。基底副变质岩中广泛分布淡色花岗岩脉体．在基底副变质岩中的淡色花岗岩脉体中发现紫苏辉石暗色麻粒岩残留体，这表明本区高喜马拉雅淡色花岗岩源岩为基底副变质岩，且基底副变质岩是在基底快速隆升降压的条件下发生缺水熔融生成的淡色花岗岩岩浆。     

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW-ESE to ENE-WSW or NE-SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite-pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near-surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite. Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline-type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km. The bulk chemical composition of the pseudotachylite veins is characterized by low SiO₂ and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high-Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two-pyroxene geothermometer. Using published data relating water solubilities in high-temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites. Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.     

Contact Metamorphic Reactions and Processes in the Mt. Tallac Roof Remnant, Sierra Nevada, California

Contact metamorphic facies in a large roof remnant within thecomposite Sierra Nevada batholith vary from actinolite hornfelsto pyroxene-hornfels facies. Hornblende-hornfels facies rockssurround most plutons. The pyroxene-hornfels facies, definedby the appearance of hypersthene in mafic rocks, occurs onlywhere two aureoles overlap. The rate of reaction of biotite plus quartz to hypersthene plusK feldspar plus water in the pyroxene-hornfels facies was controlledby the rate of escape of water. The reactants are still presentin the rocks; hypersthene was not stable where water pressurewas as high as load pressure but was able to form locally aswater vapor escaped and water pressure fell below load pressure. The reaction of epidote to andradite took place in the high-gradepart of the hornblende-hornfels facies and in the pyroxene-hornfelsfacies rocks. Hydrothermal experiments show that this reactiontakes place in the range 575 to 600 C at 2,000 bars. The highestcrystallization temperature within the hypersthene-bearing rocksis estimated to have been 750 C. The recrystallization temperatureof the clinopyroxene-bearing mafic hornfelses in the hornblende-hornfelsfacies is estimated to have been 625 to 650C, and that ofthe hornblende-bearing mafic hornfelses to have been 525 to550C. Interpretation of the fabric of the hypersthene rocks suggeststhat most of the heat was transferred to the walls by conductionand radiation. The main effect of hot magmatic vapor was toproduce a retrograde zone at the end of the recrystallizationperiod. The duration of the high-temperature phase of the metamorphismis estimated to be of the order of 10⁵ years.