Showing papers on "Phenocryst published in 1980"

Rare Gas Isotopes in Hawaiian Ultramafic Nodules and Volcanic Rocks: Constraint on Genetic Relationships

Abstract: Differences in the rare gas isotopic ratios, especially the ratios of helium-3 to helium-4 and of argon-40 to argon-36, in Hawaiian ultramafic nodules and phenocrysts in volcanic rocks indicate that the nodules and phenocrysts were derived from different sources. The isotopic ratios in ultramafic nodules are similar to those in oceanic tholeiites. The phenocrysts seem to have formed in equilibrium with source materials richer in primordial components than those of the oceanic tholeiites. Mixing between the sources is quite likely.

Crystallization history of Kilauea Iki lava lake as seen in drill core recovered in 1967-1979

Abstract: Kilauea Iki lava lake formed during the 1959 summit eruption, one of the most picritic eruptions of Kilauea Volcano in the twentieth century. Since 1959 the 110 to 122 m thick lake has cooled slowly, developing steadily thickening upper and lower crusts, with a lens of more molten lava in between. Recent coring dates, with maximum depths reached in the center of the lake, are: 1967 (26.5 m). 1975 (44.2 m), 1976 (46.0 m) and 1979 (52.7 m). These depths define the base of the upper crust at the time of drilling. The bulk of the core consists of a gray, olivine-phyric basalt matrix, which locally contains coarser-grained diabasic segregation veins. The most important megascopic variation in the matrix rock is its variation in olivine content. The upper 15 m of crust is very olivine-rich. Abundance and average size of olivine decrease irregularly downward to 23 m; between 23 and 40 m the rock contains 5–10% of small olivine phenocrysts. Below 40 m. olivine content and average grainsize rise sharply. Olivine contents remain high (20–45%, by volume) throughout the lower crust, except for a narrow ( 106 poises, and probably had the properties of a Bingham body, rather than a Newtonian fluid, by that date, which was several years after olivine removal ceased.

The nature of differentiation trends in some volcanic rocks from the Galapagos Spreading Center

Abstract: The Galapagos spreading center has produced a suite of volcanic rocks which include (1) typical ocean floor tholeiites, (2) iron- and titanium-enriched tholeiites, (3) andesites, and (4) rhyodacites. Interpretation of major element compositions determined for these rocks, associated glassy selvedges, and mineral phases suggests that the entire suite can be produced by fractional crystallization of the observed low-pressure phenocrysts. Early fractionation to extreme iron and titanium enrichment requires 50–75% crystallization of five parts plagioclase, three parts augite, and one part olivine. In the highly fractionated basalts, pigeonite replaces olivine as a phenocryst phase and is a minor phase in the fractionation. At extreme iron and titanium enrichment, titanomagnetite becomes a phenocryst phase, and fractionation produces a silica enrichment trend to andesite (79% total crystallization) and rhyodacite (87% total crystallization). Apatite occurs as a microphenocryst phase in the silicic rocks. This suite of iron- and titanium-enriched basalt and low-alumina andesite differs from the calc-alkaline suite of island arcs and continental margins. It is similar to the suite of fractionated lavas of the Galapagos Islands and Iceland, though these rocks are generally more alkaline than the Galapagos spreading center suite, which is similar to other ocean floor basalt suites though more fractionated. Magma mixing is suggested by the presence of basaltic xenoliths in the silicic rocks; the presence of high-silica glass inclusions in the glassy selvedges of the basalts, andesite, and rhyodacite; the common occurrence of reversely zoned phenocrysts in the silicic rocks; and the occurrence of some highly fractionated yet aphyric rocks. The unusual extent of fractionation for this region may be related to the size, morphology, and eruption and resupply rates of the magma chambers associated with the Galapagos spreading center. This and the regional distribution of these highly fractionated rocks suggest an important, though indirect, influence of the Galapagos hot spot.

Sm-Nd isotopic study of garnets and their metamorphic host rocks

Abstract: In this feasibility study, Sm/Nd and 143Nd/144Nd ratios have been determined on garnets and coexisting mineral and whole-rock phases from a selection of amphibolite facies rocks, granulite facies rocks and an eclogite. In two micaceous schists and an eclogite the distribution coefficient of the Sm/Nd ratio in garnet relative to other phases is in the range 2-3, i.e. comparable to values for phenocrysts in igneous rocks. In two granulite facies rocks the garnets have Sm/Nd distribution coefficients relative to other phases in the range 13-20 and have yielded precise ages.Garnet from a granulite facies shear zone from the Limpopo belt in southern Africa, of Archaean age, has yielded concordant duplicate ages of 1974±14 Ma and 1988±14 Ma. These results have been interpreted as relating to the last tectonic event in the region, long after the main gneiss-forming event. Garnet from a granulite facies xenolith from a Carboniferous vent in the Midland Valley of Scotland has yielded an age of 356±10 Ma; this is related to the onset of basic magmatism in the region.

A Regular Solution Model for Met-Aluminous Silicate Liquids: Applications to Geothermometry, Immiscibility, and the Source Regions of Basic Magmas

Abstract: Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi3O8 in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi3O8 in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.

An experimental investigation of Iceland and Reykjanes Ridge tholeiites: I. Phase relations

Abstract: Tholeiite basalts from 60° N to 65° N on the Mid-Atlantic Ridge were melted and recrystallized at atmospheric pressure in a CO2-H2 gas mixture. Seven basalts are from the Langjokull-Thingvellir volcanic zone and the Reykjanes Peninsula of Iceland and nine are from the Reykjanes Ridge. The crystallization sequence in both Iceland and Reykjanes Ridge basalts with (Total Fe as FeO)/(Total Fe as FeO+ MgO) [F/F + M] less than 0.6 is olivine, plagioclase, clinopyroxene. Chromian spinel crystallizes before plagioclase in one Iceland and one Reykjanes Ridge basalt with F/F+M less than 0.57. Chemical differences of the two groups of basalts (lower SiO2 and higher alkalis in Iceland basalts) can not simply be a result of low pressure fractional crystallization. Liquidus temperatures of the seven Iceland basalts decreases from 1,230° C to 1,170° C as the F/F+M of the rock increases from 0.52 to 0.70. The liquidus temperatures of the Reykjanes Ridge basalts are about 10° C lower than those of the Iceland basalts for the same F/F+M value. The profile of measured liquidus temperatures from 65° N on Iceland to 60° N on the Reykjanes Ridge has a minimum value at 63.2° N on the Reykjanes Ridge just south of Iceland. Model calculations of the pressure of phenocryst crystallization indicate that olivine and plagioclase in Langjokull basalts could have equilibrated between 2.0 and 6.2 kb (200 to 620 MPa). Phenocryst assemblages in Reykjanes Ridge basalts at 60° N could have crystallized together at greater than 2 kb (200 MPa) and probably less than 8 kb (800 MPa). A minimum in the equilibrium pressure of phenocryst crystallization occurs between 62.9° and 64° N and coincides with the minimum in the experimentally determined liquidus temperatures. The more extensive fractionation at low pressure in this area could be related to the shift of the Mid-Atlantic Ridge axis along the leaky transform fault from the Reykjanes Ridge to the Thingvellir volcanic zone.

A potassium-rich Alkalic Suite from the Deccan Traps, Rajpipla, India

Abstract: The Rajpipla Alkalic Suite is the most potassium-enriched group of basaltic rocks so far described from the Deccan Traps. In the same area however early tholeiitic flows and late tholeiitic dykes show the potassium-poor nature characteristic of most Deccan Trap magmas. The rocks of the alkalic suite are highly porphyritic and their major element variation can be interpreted in terms of crystal fractionation dominated by clinopyroxene. Plagioclase, which is an important phenocryst phase, has fractionated only in relatively small amounts as a result of a lack of density contrast between it and the liquids. A dyke-like form for the magma chambers in which fractionation has taken place is postulated to account for the abundance of highly porphyritic types. The Rajpipla area is also notable as being one of the few Deccan localities where rhyolites are found.

Geology, petrography, and petrology of Pinzon Island, galapagos archipelago

Abstract: Three stratigraphic units based on geologic relationships and paleomagnetic observations may be distinguished on Pinzon Island. The oldest unit is a broad shield which forms the main body of the island and was erupted during a period of reversed magnetic polarity from an area now occupied by a caldera. Subsequent activity was centered about 1.5 km to the north-northwest from vents later engulfed by the collapse of a younger caldera. The lower portion of this sequence was erupted during a period of transitional pole positions and is overlain by flows of normal polarity. Pinzon has the most diverse suite of differentiated tholeiitic rocks found in the Galapagos Archipelago. Products of eruptive cycles are preserved as sequences of tuffs and flows that have decreasing degrees of differentiation and increasing phenocryst abundance upsection. The sequences may be a consequence of tapping successively deeper levels of compositionally zoned magma chambers. Such a model is consistent with computer calculations utilizing major and trace element data for Pinzon rocks, which suggest that lavas of the island may be related by shallow-level crystal fractionation of observed phenocryst minerals.

Chromite-bearing spessartites from Kasuga-mura, Japan, and their bearing on possible mantle origin andesite

Abstract: An unusual association of chromite and hornblende was found in the spessartites of andesite composition, occurring as a dike swarm associated with a Cretaceous granite batholith. The spessartites are largely porphyritic with phenocrysts of either hornblende or augite. One dike, comprising a finegrained spessartite, exhibits distinct chilled selvages of aphanitic facies. The chromites in the fine-grained and augite-spessartites are significantly higher in Cr/ (Cr+Al) than those occurring rarely as inclusions in the phenocrystic hornblendes in the hornblende spessartite, although both are similar in Mg/ (Mg+Fe), Fe2O3, and TiO2. The phenocrystic hornblendes are titaniferous pargasite with high Mg/ (Mg+Fe), and differ in their higher octahedral Al from the groundmass hornblendes including those in the fine-grained spessartite. The crystallization sequence in the phenocrystic hornblende-bearing spessartites is Al-rich chromite, phenocrystic hornblende, and plagioclase without pyroxene, suggesting a high water content in the magma and the start of the crystallization at relatively high pressures. The finegrained spessartite from which the porphyritic spessartites have been derived by fractionation of dominant mafic minerals, has the high Mg-value and Cr content equivalent to those in primitive, undifferentiated basalts, although still andesitic in SiO2 content. Chemically similar magnesian andesites, although uncommon, found in some orogenic calc-alkalic suites may represent a magma composition in equilibrium with mantle peridotite under the condition of high water pressures.

The distribution, petrology, and genesis of nickel ores at the Juan Complex, Kambalda, Western Australia

Abstract: Massive and disseminated Fe-Ni-Cu ores in the Juan complex occur in two belts of contrasting Ni tenor that are interpreted as original sea-floor depressions in basalt flows into which separate sulfidic komatiitic magmas were extruded. Similarities in composition and coincidental areal distribution of massive and disseminated ores indicate that their emplacement was nearly simultaneous. These features, the distribution of metasomatic reaction zones at basalt-ultramafic contacts, and the presence of interpillow massive Fe-Ni sulfides in the underlying basalt, all point to the existence of abundant primary massive sulfides on top of the basalt.A magmatic genetic model is favored for the Juan ores in which massive sulfides were extruded and emplaced slightly in advance of viscous komatiitic magma rich in olivine phenocrysts, as dictated by contrasts in flow rates. The thick komatiitic flow came to rest upon massive sulfides that were partly crystalline, due to rapid marginal chilling. Residual sulfide droplets settled during flow at the base of the flow, thus forming disseminated ore.During ensuing prograde regional metamorphism, massive ore was thickened in some folds and mobilized into some faults and previously barren contacts, but overall redistribution was very limited. Metamorphism did not create new massive ore.During declining metamorphic temperatures, oxidation reactions and attendant sulfur and copper diffusion generated metamorphic magnetite along sulfide-silicate contacts, pyritic layers and lenses in massive ore, and Cu-rich sulfide stringers in wall rocks. On final cooling, sulfide ores were annealed and many tectonic surfaces were overprinted by static recrystallization of silicates and sulfides.

Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon

Abstract: Intermediate lavas from the middle Miocene McDermitt caldera complex, Nevada-Oregon, have very high FeO* (= total Fe as FeO; 9.1 to 10.2 wt %) and low MgO (0.4 to 2.1 wt %) contents at SiO 2 contents of 59.5 to 62 wt %. With the exception of significantly higher K 2 O contents (3.1 to 4.7 wt %), these lavas are compositionally very similar to icelandite (“tholeiitic andesite”) from oceanic areas. In addition to the voluminous comendite tuffs of the complex, a thin unit of densely welded crystal-rich pantellerite tuff of probable air-fall origin is exposed on the northern wall of the McDermitt caldera. The pantellerite tuff contains 1.5% aenigmatite phenocrysts. The high FeO*/MgO ratios of the icelandites and the peralkaline nature of the silicic rocks suggest differentiation under conditions of low f O 2 and f H 2 O. The existence at McDermitt of a continuous series of rocks ranging from potassic icelandite to comendite and pantellerite is consistent—as are other geologic, geochemical, and geophysical data—with the derivation of the icelandites and ultimately of the voluminous peralkaline silicic rocks of the complex from mafic magma generated from upwelling mantle material. Labile U released during the crystallization of late subvolcanic bodies of peralkaline silicic magma may have provided an appreciable proportion of the epigenetic U now present in the upper part of the caldera complex. Reducing conditions produced by the very Fe-rich glassy icelandites may have contributed to the fixation of U at the Aurora and Bretz prospects.

Chemical variation of clinopyroxene phenocrysts from the trachybasaltic lavas of Mount Etna, Sicily

Abstract: The chemical variation of clinopyroxene phenocrysts from the trachybasaltic lavas of Etna volcano is described. The phenocrysts show a limited, but distinct trend in chemical variation from calcic-augite in the hawaiites to augite in the benmoreites. The trend of this variation is unusual, being one of Mg-enrichment with differentiation of the magma. Ca shows a steady decrease in the clinopyroxenes from the hawaiites to the benmoreites. Na, however, shows little chemical variation in the pyroxenes. The trace element chemistry is briefly examined. The clinopyroxenes show well-developed oscillatory and sector zoning. The basal {11} sectors are enriched in Si and Mg and depleted in Ti, Al, and Fe relative to the {100}, {110}, and {010} prism sectors.

Zoned Pyroxenes and Amphiboles from Camptonites Near Gran, Oslo Region, Norway

Abstract: CAMPTONITE sills of Permian age, intruded into Cambrosilurian sediments, are present in the northern part of the Oslo region, immediately to the north of Gran, Hadeland. They have been considered, along with associated dykes of camptonite and other minor intrusions of maenaite, to be related to the predominantly gabbroic rocks comprising the Oslo-essexite plugs (Br6gger, I894). Porphyritic varieties of carni)tbriite are the most common, containing phenocrysts of concentrically zoned clinopyroxene and concentrically zoned brown amphibole; but, aphyric varieties with abundant brown amphibole also occur. A second amphibole (actinolite) forms thin, patchy, green overgrowths to some phenocryst or matrix brown amphiboles. The pyroxenes range in composition from diopside to sahlite. They contain up to 0.7 % Cr203 in the most Mg-rich zones and up to 0.6 % Na20 in the most Fe-rich zones. TiO2 and Al~O3 are in the ranges 0.5-3.8 % and 3.o-9.2 % respectively. These compositions are similar to pyroxenes from other camptonites and alkali basaltic and alkali gabbroic rocks. 'Normal ' zoned phenocrysts, in which an outer pinkish zone surrounds a colourless core in thin section, have more Fe, Ti, and A1, and less Mg and Si in the outer zone. In 'reversed' zoned crystals, the slightly pinkish core contains more Fe, Ti, and A1, and less Mg and Si. Oscillatory zoning, restricted to three concentric zones (pinkish-colourless-pinkish) show the same chemical changes with colour variation. In terms of end-member molecules, substitutions involving CaTiA1206, CaAI2SiO6 plus CaFeSi206 replacing CaMgSi206 are responsible for the zoning. The majority of the zoned brown amphiboles are kaersutite following the criteria of Leake (1978). Those zones with Ti 940 ~ and at probably greater depths (7-i 5 km) than crystallization of nearby exposed gabbroic plugs (Oslo essexites). A mechanism Of alternate crystallization of pyroxene and kaersutite in response to changes in PH~O, with some partial resorption of the non-crystallizing phase, is suggested to account for the observed petrographic features and oscillatory zoning in both minerals. The actinolite is a product of late-stage metasomatic activity.

Trace elements in continental-margin magmatism: Part I. Trace elements in the Clarno Formation of central Oregon and the nature of the continental margin on which eruption occurred: Summary

Abstract: The Clarno Formation is an Eocene to lower Oligocene volcanic and volcaniclastic formation in central Oregon It was apparently formed in response to subduction of Pacific Ocean crust under the western margin of North America Clarno flow rocks, which constitute at least 50% of the formation, are compositionally intermediate between flow rocks typical of continental margins and those typical of island arcs The dominant rock type is andesite, in a range from rhyolite to basalt, and the SiO 2 frequently distribution is unimodal about a mean of 60% The principal phenocryst in the basalts and andesites is clinopyroxene Abundances of lithophile elements (K, Rb, and Ba) in the basalts are very similar to average abundances in calc-alkalic basalts of island arcs Lithophile abundances in Clarno andesites, however, are generally greater than those of island-arc andesites and less than those of andesites from continental margins The Clarno Formation may be inferred to have formed on a crust intermediate between oceanic and continental; a thickness of 20 to 30 km can be estimated from K 2 O contents and also from Rb-Sr relationships

Petrology of Ti Point Group, Northland, New Zealand

Abstract: Lavas of the late Miocene Ti Point Group are found as scattered flow remnants, a dike, and a sill Most are basalts with olivine phenocrysts set in a groundmass of clinopyroxene and plagioclase One flow remnant is a glassy andesite and the relationship of this rock to the basalts is unknown Together they are interpreted as the remains of a basaltic lava field which lay along the east coast of Northland between Ti Point and Mangawhai The basalts belong to distinctive alkali-rich and alkali-poor groups similar to the Quaternary volcanic fields of Auckland and Northland

Pyroxenes of the Fen alkaline complex, Norway

Abstract: Compositional data are presented for pyroxenes from the undersaturated rocks of the Fen alkaline complex. Four groups of pyroxenes are present: (l) Al-Na diopsides rich in CaAlrSiOu, forming the cores of phenocrysts in damkjernite; (2) Ti-Al salites rich in CaTiAlrO., occuring as rims to phenocrysts and as strongly zoned discrete crystals in damkjernite and vibetoite; (3) an acmitic-hedenbergite series found in urtites, ijolites, and silicocarbonatites; (4) an acmitic series characteristic only of the later stages of urtite crystallization. Damkjernites are considered to have evolved at high temperatures over a very wide pressure range and the closely related vibetoites at relatively lower pressures and temperatures under HrO-rich conditions. Other rocks in the complex formed at low pressures under COr-rich conditions, possibly over a range of temperatures. Pyroxene and mica compositions indicate that urtite is not simply related to ijolites by fractional crystallization and that slvites are unlikely to be the end product of extensive crystallization of the ijolite-silicocarbonatite series. The Fen complex is considered to represent the root ofan undersaturated volcano and the rocks present are the result of the crystallization under different pressures, temperatures, and volatile conditions of several batches of the same undersaturated (nephelinite or alkali basalt) magma. Thus none of the rocks currently exposed at Fen are related by in-situ ftactional crystallization of a single batch of magma.

Petrology of the Volcanic Rocks of Saba, West Indies

Abstract: Saba is composed of basic and intermediate members of the calc alkaline series with a broad resemblance to volcanic rocks on other islands of the northern Lesser Antilles. The most abundant rock-type is hornblende andesite with relatively high potassium and associated elements. Notable petrographic features are the persistence of magnesian olivine into relatively siliceous whole-rock composition and the prevalence of hornblende even in the more basic rocks. Quartz xenocrysts also occur in the basalts Clinopyroxene phenocrysts are of an gite composition but groundmass grains are sometimes in the range subcalcic augite to magnesian pigeonite. Disequilibrium features in the petrography, together with anomalies in the chemical trends may be due to incorporation of xenocrysts or perhaps in extreme instances to the hybridisation of contrasting magmas.

Rare gas isotopes in Hawaiian ultramafic nodules and volcanic rocks: constraint on genetic relationships

Abstract: Differences in the rare gas isotopic ratios, especially the ratios of helium-3 to helium-4 and of argon-40 to argon-36, in Hawaiian ultramafic nodules and phenocrysts in volcanic rocks indicate that the nodules and phenocrysts were derived from different sources. The isotopic ratios in ultramafic nodules are similar to those in oceanic tholeiites. The phenocrysts seem to have formed in equilibrium with source materials richer in primordial components than those of the oceanic tholeiites. Mixing between the sources is quite likely.

Rapakivi texture from the O’Leary Porphyry, Arizona (U.S.A.)

Abstract: The rhyodactic O’Leary Porphyry which forms the Pleistocene (0.233±0.37 m.y.) volcanic domes of O’Leary Peak and Darton Dome in the San Francisco Volcanic Field (northern Arizona, U.S.A.) contains sanidine phenocrysts with oligoclase mantles (rapakivi texture). Rapakivi texture occurs worldwide in silicic rocks of many ages and has been attributed to various igneous and metamorphic processes. The O’Leary Porphyry contains both mantled and unmantled sanidine (both are Or63–69 Ab30–36An1), oligoclase and quartz phenocrysts, labradorite (An53Ab45Or2) and kaersutite xenocrysts and andesite xenoliths. The compositional range of oligoclase is the same (An11–26Ab70–80Orr–10) for the rapakivi mantles, the oligoclase phenocrysts, and the oligoclase crystals poikilitic within sanidines. Most mantles are discontinuous. The sanidine appears to have been resorbed prior to mantling. Experimental melting studies on the O’Leary Prophyry show that, for a 15 wgt.% water system, plagioclase crystallized prior to sanidine and quartz crystallized last. The O’Leary Porphyry, although inhomogeneous, plots on a Q-Or-Ab-An diagram well within the plagioclase stability field. Poikilitic plagioclases within sanidines further support crystallization of plagioclase prior to sanidine in the O’Leary Porphyry. Exsolution of a ternary feldspar to form a plagioclase mantle is the most commonly accepted igneous theory of rapakivi texture formation but has been eliminated as the origin of the O’Leary Porphyry rapakivi. Petrologic models by Tuttle and Bowen and by Stewart are rejected for the O’Leary rapakivi because of inconsistencies with the O’Leary occurrences. Two theories are viable for the O’Leary rapakivi texture. First, is a decrease in water vapor pressure which would enlarge the plagioclase stability field possibility causing mantling of metastable sanidines. The second and preferred theory is that of an addition of sodium and calcium by basification (chemical assimilation without melting) of the xenoliths within the O’Leary Porphyry. This would move the bulk composition of the melt into the plagioclase field possibly resulting in crystallization of plagioclase on sanidine crystals. Diffusion of sodium and calcium from the xenoliths to sanidine would result in mantling only those crystals near to the xenoliths. Later, convection would result in distribution throughout the melt of rapakivi, unmantled sanidines, and xenolithic kaersutite as is seen in the porphyry. Basic xenoliths are extremely common in rapakivi-bearing rocks. Those within the O’Leary Porphyry are andesitic and show resorption, and in some areas of O’Leary Peak itself, have been drawn out into schlieren.

The Neranleigh-Fernvale beds of southeastern Queensland : petrology, sedimentology, structure metamorphism, and tectonic evolution

Abstract: The origin and geological evolution of the Neranleigh-Fernvale Beds and other units of the Brisbane Metamorphics in the Beenleigh and D'Aguilar Blocks of southeastern Queensland were examined in this study. The Neranleigh-Fernvale Beds consist of quartzo-feldspathic greywackes, conglomerates, shale, chert and jasper, and basaltic volcanics. The sedimentary associations of the Beenleigh Block can be described in terms of a single deep-sea fan system. Coarse, thickly bedded greywackes and conglomerates dominate the southern and western areas of the block. They represent the inner fan association, and grade east and northeast into the channelized suprafan association (dominated by sandstone and thin shale interbeds) and the outer depositional lobes (dominated by thinly bedded sandstone-shale alternations) of the mid-fan. Outer fan deposits consisting of thinly bedded shale, siltstone, minor sandstone, and chert, occupy the northern end of the Beenleigh Block. Sediment was transported from the west. The southern D'Aguilar Block is peripheral to the Beenleigh Block fan system, but does contain some greywackes comparable to those in the Beenleigh Block. A typical basin-floor association (chert-jasper-shale-basalt) is present in parts of the southern D'Aguilar Block. Oolite-bearing acid volcanolithic arenites, which are absent from the Beenleigh Block, occur in the southern D'Aguilar Block, and indicate other, minor sediment supply systems also operated. The Beenleigh Block has been affected by two major deformational events. The first phase produced usually tight to isoclinal mesoscopic and macroscopic folding and an axial-plane foliation dipping west and striking north-northeast. The foliation is a slaty cleavage in pelitic rocks. Transposition has probably occurred mesoscopically and macroscopically, but is most obvious on a mesoscopic scale in pelitic rocks. The average stratigraphic thickness of sediment in the Beenleigh Block is of the order of 2-3.5 km. Second-generation deformation in the Beenleigh Block has produced open macroscopic and mesoscopic folding of first-generation structures. The axial-plane foliation is a crenulation cleavage in pelitic rocks, and dips to the east and strikes north-northwest. Transposition is not associated with this deformation. First-generation deformation in the southern D'Aguilar Block is comparable in style and patterns of preferred orientation to that in the Beenleigh Block. The dominant second-generation structures do not, however, compare with those in the Beenleigh Block. They are characterized by a shallowly dipping foliation. The first-generation slaty cleavage is almost completely transposed into parallelism with this foliation, which largely defines the Bunya Phyllite. Strati graphic significance cannot be attributed to this foliation, and the Bunya Phyllite represents a more deformed equivalent of the Neranleigh- Fernvale Beds. The Rocksberg Greenstones are also considered to be laterally equivalent to the Neranleigh-Fernvale Beds. The first-generation structural elements are widespread, and suggest that tectonic shortening by compression orthogonal to the main north-northwesterly structural trends has occurred. The Neranleigh-Fernvale greywackes principally consist of plagioclase, quartz, and K-feldspar. Acid-volcanic, granitic, and shale fragments are the principal lithic components. Hornblende, biotite, muscovite, ilmenite, sphene and zircon are the main accessory grains. The greywackes are mostly medium to very coarse in mean grainsize, and do not contain a primary matrix. Igneous rock types in the conglomerates are representative of the source material for the greywackes. They include acid-volcanic and granodioritic-granitic rocks which were probably derived from a high-level volcanoplutonic complex, and have mineralogical and chemical affinities with calc-alkaline, active continental-margin suites. The greywackes are chemically characterized by high and S1O2 and A12O3 (average 70.6 and 14.2 wt. % respectively) and high Na20 and K20 levels (3.14 and 3.46 wt. % respectively). Fe in the greywackes is largely in the ferrous state. In terms of Q-or-ab normative compositions, the Neranleigh-Fernvale greywackes are comparable to greywackes in other "eugeosynclinal" suites. The Neranleigh-Fernvale cherts and jaspers contain relict radiolaria, and are composed of granulo-chalcedony or quartz. They are considered to be recrystallized radiolarian oozes, and are not volcanogenic in origin. Chemically the cherts are composed of 96% or greater S102. Total Fe as Fe2O3 in the jaspers is less than 1.0%. P, Ba, and Cu are the principal minor and trace elements. They, as well as Ni, Zn, Pb and other metals are interpreted as being biogenic in origin. Basaltic volcanics are interbedded with the Neranleigh-Fernvale sediments. They are light or dark green in colour, and are either massive, or have pillow or "pahoehoe"-like structures. The volcanics were originally hyaline or partly hyaline, and are clearly submarine extrusions. Microphenocrysts of clinopyroxene, plagioclase, and iron oxide are present in rare cases. The volume percentage and size of rare vesicles indicate a depth of extrusion of 3.2-3.5 km for the volcanics. Geochemical parameters suggest that the volcanics are tholeiitic, and have ocean-floor basaltic affinities. Relict primary features of the Rocksberg Greenstones in the D'Aguilar Block suggest that they were originally crystal and agglomeratic lavas, tuffs, and intrusive sills. They commonly contain large relict augite phenocrysts. The actinolite-bearing metamorphic assemblages in the groundmass are similar to the metamorphic assemblages in the Neranleigh-Fernvale metabasalts. The groundmass was probably also hyaline. Geochemical parameters suggest that the Rocksberg Greenstones are tholeiitic, and have ocean-floor basaltic affinities. The Rocksberg Greenstones and the Neranleigh-Fernvale basalts have comparable average A12O3 levels which are slightly lower than average ocean-floor basalts. Both groups of volcanics are therefore closely comparable in geochemistry and tectonic environment in which they formed. No significant difference in time of emplacement is attributed to the two units. The principal metamorphic parageneses in the Neranleigh-Fernvale greywackes contain phengite, stilpnomelane, actinolite, epidote, chlorite, calcite, albite and sphene. Prehnite and pumpellyite are rare. The greywackes are only partially recrystallized. Stilpnomelane formed by the addition of Fe2+, Mn2+, SiO2 and H2O to feldspar. The Fe and Mn were derived from the breakdown of detrital ilmenite. ………………………………………..