Low-pressure Fractional Crystallization Research Articles

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge basalts (MORBs) from the North Atlantic. Analog models of natural glasses and associated phenocrysts show that several possible parental magmas may undergo low pressure fractional crystallization involving olivine and spinel, followed by plagioclase, and then by augite. The phenocryst phase assemblages in natural deep-sea basalts are closely correlated with the major element compositions of their associated quenched glasses, and the projections of these glasses on the Oliv-Cpx-Qtz pseudoternary correspond to the 1-atmosphere phase boundaries and reaction points defined by laboratory experiments. Comparison of natural phenocryst's with experimental phases indicates that the augites preserved in moderately fractionated MORB from the FAMOUS area may have formed at or near the ocean floor and need not be relics of high pressure processes.

Ferrobasalts from the Spiess Ridge segment of the Southwest Indian Ridge

Highly vesicular, microporphyritic basaltic rocks have been dredged from the slow-spreading Spiess Ridge segment of the Southwest Indian Ridge. All the samples recovered are hyalocrystalline with plagioclase, clinopyroxene and olivine as phenocryst and microphenocryst phases. Titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples. In terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions(e.g. FeO*=10.3−14.2%;TiO 2=2.0−3.4%;K 2O=0.50−1.1%;MgO=6.0−3.5%;Zr=160−274ppm;Nb=14−32ppm) and can be classified as ferrobasalts. Isotopic and incompatible element ratios of the lavas(e.g. 87Sr/ 86Sr=0.70325−0.70333;Zr/Nb=8.4−11.3;Y/Nb=2.3−1.4) indicate their strongly “enriched” nature (see also Dickey et al. [6]). Quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low-pressure fractional crystallisation of plagioclase, clinopyroxene and minor olivine and titanomagnetite. The range in composition can be accounted for by up to 65% fractional crystallisation. We suggest that the extreme differentiation of the Spiess Ridge lavas is related not to spreading rate, but to rate of magma supply. The basaltic melts appear to have evolved in a newly established zone of magmatic activity, associated with the most recent northward jump of the Bouvet triple junction, where they were effectively isolated from significant admixture of primitive magmas.

Petrology and geochemistry of the Kamloops Group volcanics, British Columbia

The Kamloops Group is an alkali-rich calc-alkaline volcanic suite of Early to Middle Eocene age, widespread in south-central British Columbia. Rock types in the suite range from high-K basalt through andesite to rhyolite. The suite is characterized by relatively high K2O, Sr, and Ba, but low Zr, Ti, and Ni concentrations, only moderate Ce enrichment, and little or no Fe enrichment. Initial ratios 87Sr/86Sr are about 0.7040 in the western half, and about 0.7060 in the eastern half of the study area. No difference in chemistry or mineralogy marks this sharp transition. Chemically similar suites include the Absaroka–Gallatin suite in Wyoming and the lower San Juan (Summer Coon) suite in Colorado. The content of K2O at 60% SiO2 increases regularly eastward across southern British Columbia. The chemical data support the subduction-related continental arc origin of the Kamloops Group volcanics.The volcanic rocks consist in the main of augite–pigeonite andesites ranging from 52 to 62% silica, with subordinate quantities of olivine–augite–pigeonite basalt and biotite rhyodacite and rhyolite. The andesites and basalts were derived by a combination of low-pressure fractional crystallization, higher pressure fractional crystallization, and variable parental magmas, whereas low-pressure fractional crystallization of plagioclase, biotite, and apatite from parental basalt and andesite produced the rhyolites. The parental magmas were basalts and basaltic andesites with high K, Sr, and Ba. The primary source of these magmas is inferred to have been an alkali-enriched hydrous peridotite with neither plagioclase nor garnet present in the residuum.

Petrography and geochemistry of basaltic rocks from the Conrad fracture zone on the America-Antarctica Ridge

Intrusive and extrusive basaltic rocks have been dredged from the Conrad fracture zone (transecting the slow-spreading America-Antarctica Ridge). The majority of rocks recovered are holocrystalline with the dominant mineral assemblage being plagioclase plus clinopyroxene with or without minor Fe-Ti oxides (olivine occurs in only three samples) and many of the samples show evidence of extensive alteration. Secondary minerals include chlorite, actinolite, K- and Na-feldspar, analcite and epidote. In terms of bulk chemistry the rocks are characterized by their generally evolved and highly variable compositions (e.g. Mg *=0.65−0.35;TiO 2=0.7−3.6%;Zr=31−374ppm;Nb=<3−21ppm;Y=17−96ppm;Ni=100−9ppm ), but with respect to the immobile incompatible element ratios (e.g. Zr/Nb, Y/Nb, La/Sm) are similar to “normal” or depleted mid-oceanic ridge basalts. Quantitative major and trace element modelling indicate that most of the variation observed can be attributed to low-pressure fractional crystallization of plagioclase plus clinopyroxene in approximately equal proportions with or without minor Fe-Ti oxides. The range in composition can be accounted for by up to 76% fractional crystallization. Although ferrobasalts have not frequently been associated with slow spreading ridges, the extreme differentiation observed in the Conrad fracture zone basalts implies some additional constraint other than spreading rate on the formation of ferrobasalt and reaffirms the importance of extensive crustal differentiation during the production of this basalt type.

Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt

The Western Mediterranean Jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts. In this paper we report analyses of rare earth elements (REE), transition elements (Ni, Co, Cr, V, Sc) and high field strength elements (Ti, P, Y, Zr, Ta, Hf) for metabasalts from representative ophiolite outcrops in the Eastern Alps, Corsica and the Northern Apennines (Liguria). The chemical characteristics of the metabasalts range from “normal” to “transitional” mid-ocean ridge basalt (MORB). Most chemical variation in the metabasalts from the different areas can be explained by low-pressure fractional crystallization, by differences in degree of partial melting, and by minor chemical heterogeneities of the source, but the “transitional” MORB characteristics of some metabasalts from Corsica (Balagne) might reflect formation from a source with different mineral and chemical composition. The estimated REE pattern of the source of the Liguria-type metabasalts corresponds to the pattern for certain Ligurian ultramafic rocks, which might therefore represent the residue from extraction of some ophiolitic lavas. While the Liguria-type ophiolitic metabasalts might represent products of a “normal”, for instance, oceanic ridge, the “transitional” metabasalts might be lavas erupted, during the early stages of opening of a small ocean basin, or along the continental margin of a larger ocean basin.

Rare earth geochemistry of Lewisian granulite-facies gneisses, northwest Scotland: Implications for the petrogenesis of the Archaean lower continental crust

Rare earth element (REE) data, together with data for major elements and 14 other trace elements, are presented for ultramafic, mafic, intermediate, tonalitic, trondhjemitic, anorthositic and microline gneisses, representative of the range of rock types making up the 2.9-b.y. Lewisian granulite complex of northwest Scotland. The data are used to constrain petrogenetic models for the Archaean lower crust. Ultramafic gneisses have flat REE patterns with 3–5 times chondrite abundance. The more Fe-rich mafic gneisses show slight light-REE enrichment, range up to 40 times chondritic and some have negative Eu anomalies. Intermediate gneisses have more fractionated REE distributions (Ce N/Yb N= 4−25) but with rather constant heavy REE. Tonalitic gneiss REE patterns are also strongly fractionated, show variable heavy-REE depletion and have positive Eu anomalies. REE patterns of trondhjemitic gneisses are very strongly fractionated (Ce N/Yb N up to ∼ 300), show strong heavy-REE depletion and most have marked positive anomalies. Anorthosites and microcline gneisses have similar REE distributions to the trondhjemites in spite of different major element compositions. Modelling of the REE and other trace element patterns of processes such as fractional crystallisation and partial melting suggests that whereas the mafic gneisses can be related by low-pressure fractional crystallisation, the more silicic gneisses can only be related by high-pressure partial melting of a mafic source. Most of the gneisses represent liquid compositions; few can be regarded as cumulates or the residues of partial melting. The computed average Lewisian granulite does have a positive Eu anomaly, but this is imparted by the more fractionated tonalites and trondhjemites and not by the more mafic components of the gneiss complex. Elements such as K, Rb, Cs, Th and U are removed from the lower crust by a fluid (CO 2-rich) not a melt phase during granulite-facies metamorphism. The Archean lower crust is more silicic than implied by the andesite model for continental growth. The most suitable tectonic environment to account for the generation and intermixing of the different low-pressure and high-pressure magmas or rock types (and the presence of high-grade metasediments) would be one similar to the Cordilleran belt of South America. Underplating by subduction zone or mantle derived liquids may be a significant process in Archaean continental growth.

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

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.

Geochemistry and petrogenesis of late proterozoic volcanic rocks from north-western Africa

The Upper Proterozoic volcanism of northwestern Africa is characterized by the predominance of calc-alkaline rocks. Volcanics with tholeiitic affinities and alkali basalts are rare. The geochemistry and the relative proportions of calc-alkaline rocktypes in the Silet zone (Algeria) and the Ouarzazate formation (Morocco) are similar to those of recent island arc suites where basalts are most abundant while in the Tassendjanet and Gara Akofo zones (Algeria) they resemble contintal margin volcanic suites with a predominance of andesites. The volcanic rocks have undergone low-grade metamorphism which strongly affected alkali and alkali-earth elements and also to a smaller degree, the less mobile elements such as REE, Zr, Hf, Nb, and P. The geochemistry of the calc-alkaline rocks point to a complex origin involving low-pressure fractional crystallization, crustal contamination and derivation from a source already enriched in LILE.

Halogens in the mantle beneath the North Atlantic

F, Cl and Br contents of tholeiitic volcanic glasses dredged along the Mid-Atlantic Ridge from 53° to 28° N, including the transect over the Azores Plateau, are reported. The halogen variations parallel those of 87 Sr/ 86 Sr, La/Sm or other incompatible elements of varying volatility. The latitudinal halogen variation pattern is not obliterated if only Mg-rich lavas are considered. Variations in extent of low-pressure fractional crystallization or partial melting conditions do not appear to be the primary cause of the halogen variations. Instead, mantle-derived heterogeneities in halogens, with major enrichments in the mantle beneath the Azores, are suggested. The Azores platform is not only a ‘hotspot’ but also a ‘wetspot’, which may explain the unusually intense Azores volcanic activity. The magnitude of the halogen and incompatible element enrichments beneath the Azores appear strongly dependent on the size of these anions and cations, but independent of relative volatility at low pressure. The large anions Cl and Br behave similarly to large cations Rb, Cs and Ba, and the smaller anion F similarly to Sr and P. Processes involving crystal and liquid (fluid and/or melt), CO 2 rather than H 2 O dominated, seem to have produced these largescale mantle heterogeneities. Geochemical ‘anomalies’ beneath the Azores are no longer apparent for coherent element pair ratios of similar ionic size. Values of such ‘unfractionated’ coherent trace element ratios provide an indication of the mantle composition and its nature before fractionation event (s) which produced the inferred isotopic and trace element heterogeneities apparently present beneath the North Atlantic. The relative trace element composition of this precursor mantle does not resemble that of carbonaceous chondrites except for refractory trace element pairs of similar ionic size. It is strongly depleted in halogens, and to a lesser extent in large alkali ions Rb and Cs relative to refractory Ba. These relative depletions are comparable within a factor of 5 to Ganapathy &amp; Anders’s estimates for the bulk Earth, with the exception of Cs. There is also evidence for removal of phosphorus into the iron core during its formation. With the exception of San Miguel, alkali basalts from the Azores Islands appear to have been derived from the same mantle source as tholeiitic basalts from the ridge transect over the Azores Platform but by half as much degree of partial melting. The Azores subaerial basalts seem to have been partly degassed in Cl, Br and F, in decreasing order of intensity. A working model involving metasomatism from release of fluids at phase transformation during convective mantle overturns is proposed to explain the formation of mantle plumes or diapirs enriched in larger relative to smaller halogen and other incompatible trace elements. The model is ad hoc and needs testing. However, any other dynamical model accounting for the 400 -1000 km long gradients in incompatible trace elements, halogens and radiogenic isotopes along the Mid-Atlantic Ridge should, at some stage, require either (1) some variable extent of mixing or (2) differential migration of liquid relative to crystals followed by re-equilibration (or both), as a diffusion controlled mechanism over such large distances is clearly ruled out, given the age of the Earth.

Rare-earth element geochemistry of Madras granulites

Archaean granulites from the type charnockite locality of Pallavaram, Madras City, Tamil Nadu, southern India consist of a bimodal suite of basic and silicic orthogneisses, associated with metasedimentary gneisses (khondalites). Charnockite is the dominant rock type. Basic granulites display a tholeiitic trend of strong iron enrichment accompanied by an increase in the concentration of the rare earth elements (REE), and the development of appreciable negative europium anomalies. These trends are considered to reflect low pressure fractional crystallisation of pyroxene-plagioclase assemblages under conditions of lowf O 2. Ultramafic granulites may represent the cumulus material of such a process. The silicic granulites (charnockites) are generally K2O rich and in marked contrast to the low K2O (tonalitic) silicic gneisses which dominate most granulite facies terrains. Their REE abundances, however, embrace the complete range of patterns observed in both K-rich and K-poor Archaean gneisses. The presence of a large number of pre-granulite facies potassic pegmatites in the area suggests metasomatism of an originally less potassic suite of rocks. Trace element considerations lead to a model whereby metasomatism and partial fusion of silicic gneisses in the terrain preceded the granulite facies metamorphic event. This sequence of events is best related to fluctuations in the composition of metamorphic fluids in the lower crust.

Geochemistry of calc-alkaline volcanic rocks from southeastern Iran (kouh-e-shahsavaran)

The Upper Tertiary to Quaternary volcanic complex of Kouh-e-Shahsavaran in southeastern Iran is composed of calc-alkaline rocks of island are type (high-alumina basalts, basic andesites, andesites and dacites) even though it was emplaced on the continental basement. The volcanic rocks of the complex are genetically related and were probably derived by low-pressure fractional crystallization of high-Al basalts. The anomalously high content of Sr in some rocks probably reflects an accumulation of plagioclase. The trace element data are consistent with the origin of the parental magma by partial melting of an “enriched” upper mantle peridotite.

Rare earth element abundances in a thick, layered komatiite lava flow from Ontario, Canada

Rare earth element abundances have been measured in pyroxenitic (19.6% MgO) to gabbroic (7.7% MgO) rocks from the upper part of a thick, layered komatiite lava flow (Fred's Flow) in Munro Township, Ontario. This flow apparently erupted as a highly basic liquid which subsequently differentiated into layers of ultramafic cumulate rocks and a basaltic residual liquid. The analyzed rocks have compositions and spinifex or equigranular textures interpreted to indicate that they represent the complete range of liquids that were present during the differentiation of the lava. All the analyzed rocks are depleted in light REE, and also exhibit a slight depletion of Yb and Er relative to Gd and Dy. Chondrite-normalized Ce and Yb abundances range from 3.2 to 7.8 and 5.1 to 9.7 respectively. Proportions of fractionating minerals were estimated using a major element petrological mixing program and petrographic data. REE modeling based on these results indicates that the dominant process relating the samples is low-pressure fractional crystallization of olivine, followed at lower temperatures by clinopyroxene and plagioclase. Except for Eu, correspondence between observed and calculated REE abundances obviates any need to appeal to processes of major REE redistribution during diagenesis and low-grade metamorphism. Major differences in REE patterns of other ultramafic and mafic komatiitic lava flows [6,11], therefore, probably reflect different episodes of partial melting and/or differences in mantle source composition. The consistency of the REE in the layered flow, however, supports the concept that mafic komatiites can also be derived from ultrabasic parental magmas by low-pressure fractional crystallization. The light-REE-depleted patterns of these komatiites resemble those of modern MORB, suggesting that the mantle source of the komatiites had undergone a previous melting episode.

Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence

Research Article| January 01, 1978 Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence PETER W. LIPMAN; PETER W. LIPMAN 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar BRUCE R. DOE; BRUCE R. DOE 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar CARL E. HEDGE; CARL E. HEDGE 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar THOMAS A. STEVEN THOMAS A. STEVEN 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information PETER W. LIPMAN 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 BRUCE R. DOE 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 CARL E. HEDGE 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 THOMAS A. STEVEN 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 967182Federal Center, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1978) 89 (1): 59–82. https://doi.org/10.1130/0016-7606(1978)89<59:PEOTSJ>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PETER W. LIPMAN, BRUCE R. DOE, CARL E. HEDGE, THOMAS A. STEVEN; Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence. GSA Bulletin 1978;; 89 (1): 59–82. doi: https://doi.org/10.1130/0016-7606(1978)89<59:PEOTSJ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Two distinct suites of igneous rocks occur within the San Juan volcanic field: an Oligocene suite of predominantly intermediate-composition lavas and breccias, with associated silicic differentiates erupted mainly as ash-flow tuffs, and a Miocene-Pliocene bimodal suite of silicic rhyolites and mafic alkalic lavas.The Oligocene volcanism, probably related to subduction along the western margin of the American plate, has chemical and isotopic characteristics indicative of complex interactions with Precambrian cratonic lithosphere. It also appears to record the rise, differentiation, and crystallization of a large composite batholith beneath the San Juan field. The earliest intermediate-composition lavas and breccias have major- and minor-element compositional patterns indicative of high-pressure fractionation and are relatively nonradiogenic in both Pb and Sr, suggesting significant interaction with lower crust of the American plate. The more silicic ash-flow tuffs show compositional evidence of low-pressure fractional crystallization and are more radiogenic in Pb and Sr — features thought to indicate significant shallow residency for the magmas and interaction with upper crust. Especially radiogenic Pb-isotope compositions of some of these rocks may reflect interactions between the magmas and convecting meteoric water rich in leached Pb, a process thought to have been even more important in forming associated hydrothermal ore deposits. Ore leads tend to be more radiogenic than associated rock leads.Many of the Miocene-Pliocene basaltic lavas seem to be mantle-derived lavas, similar to those of oceanic islands, but some anomalous xenocrystic basaltic andesites, containing relatively nonradiogenic lead, may have been slightly contaminated by lower crustal components. Rhyolitic lavas and intrusions of the bimodal suite are also nonradiogenic in Pb and Sr, in comparison with the Oligocene rhyolites, and do not appear to have interacted with Precambrian upper crust, probably because they erupted largely through the subvolcanic batholith. The Miocene-Pliocene rhyolites are best interpreted as partial melts of lower crust, with the thermal energy to initiate magma generation provided by concurrent basaltic volcanism. This content is PDF only. Please click on the PDF icon to access. 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A geochemical study of island-arc and back-arc tholeiites from the Scotia Sea

87Sr/ 86Sr and 143Nd/ 144Nd ratios, REE and selected minor and trace elements are presented and compared for present-day volcanic rocks in the Scotia Sea. Tholeiitic basalts from the South Sandwich Islands show widely ranging contents of some lithophile elements, e.g. K 2O (0.09–0.55%) and Rb (1.55–14.2 ppm), but fairly constant Na 2O and Sr. Total REE contents range from about 4–20 times chondritic abundances with significant light-REE depletion and both positive and negative Eu anomalies. The variations in minor and trace element abundances are consistent with low-pressure fractional crystallization of plagioclase and clinopyroxene but only minor amounts of olivine. The 87Sr/ 86Sr and 143Nd/ 144Nd ratios of the parental magmas are thought be 0.7038–0.7039 and 0.51301–0.51314 respectively, and indicate derivation of at least some 87Sr from subducted ocean crust. The back-arc tholeiites in the Scotia Sea have lower 87Sr/ 86Sr ratios (0.7028–0.7033), similar 143Nd/ 144Nd ratios (0.51305) and are variably light-REE-enriched (Ce N/Yb N= 1.0–1.6) . Total REE contents are comparable to those of the South Sandwich Islands tholeiites.

The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie, Western Australia

Two major rock sequences, the Lower and Upper Greenstones, separated by an unconformity and associated conglomerate, are exposed in a narrow (5–30 km) elongate (130 km) greenstone belt in the Lawlers—Mt Keith area of the Eastern Goldfields of Western Australia. The Upper Greenstones, which contain zones of significant nickel sulfide mineralization, appear to have been emplaced in a graben which cuts across folded strata of the Lower Greenstones. Exposed portions of the Lower Greenstones consist of over 90% mafic and less than 10% felsic rocks, whereas the Upper Greenstones consist of approximately one third mafic and two thirds felsic rocks. The mafic rocks of the latter sequence all appear to belong to the komatiitic rock series. ∗∗ ∗∗ Using the revised definition of Arndt et al., 1977. They are divided into two groups of flows and thin sills ranging in composition from peridotite to basalt and a number of strata-bound podiform dunitic bodies which are regarded as feeder chambers for some of the overlying flows. Mafic rocks of the Lower Greenstones include Fe-rich tholeiitic basalts, gabbros, komatiitic ∗∗ ultramafic rocks and some basalts which appear chemically to be transitional between the two rock series. At Yakabindie in the centre of the Lawlers—Mt Keith belt, Western Australia, the nickel sulfide mineralization occurs mainly within the dunite pods as large (in excess of 70·10 6 tonnes) low grade (approx. 0.6 wt.% Ni) zones composed largely of disseminated pentlandite. Textures in much of the ore suggest that magmatic sulfides were trapped interstitial to olivine grains, although textures and chemical variations in some of the ore indicate that metals and sulfides have been redistributed during metamorphism or alteration of the host rocks. Chemical variations within the komatiites are consistent with their being the result of low pressure fractional crystallization of a liquid containing in excess of 33 wt. % MgO. In order to produce such a magnesian liquid, it is postulated that the source region of the mantle must have been melted twice, once to give rise to a basaltic liquid and again to give rise to the magnesian, komatiitic liquid. This would require diapiric ascent of mantle material from a depth of perhaps 200–250 km. Consideration of the melting curves of sulfides and mantle periodotite with pressure suggests that sulfides might have become concentrated at these depths, thus accounting for their almost universal association, in Western Australia, with very magnesian komatiites.

Petrology and geochemistry of basaltic rocks of the Lau Basin

The Lau Basin is a marginal sea, located between the Tonga and Lau Ridges, in the southwestern Pacific. The basin is on the “inner” or concave side of the Tonga Trench-Arc system and is situated above the deep seismic zone dipping westward from the Tonga Trench. The Tonga Trench-Arc system is undoubtedly located above a zone of crustal shortening as evidenced by the deep seismicity and vulcanism. However, the geological and geophysical data give strong support to the contention that the Lau Basin has formed by crustal dilation. Rocks dredged from ridges and seamounts in the basin are sub-alkaline basalt. The average major element composition of least altered samples is: SiO 2 48.8%, TiO 2 1.2%, K 2O 0.18%, P 2O 5 0.08%, H 2O+ 0.30%, Fe III/Fe II = 0.26,CaO/Al 2O 3 = 0.77. The data for Lau Basin basalt (LBB) show close similarity to data of typical oceanic ridge basalt (ORB). Trace element abundances (ppm): Ni 160, Cr 390, Sr 100, Ba < 31, Rb < 1 also resemble ORB values. K/Rb in a least altered and unfractionated sample is 860, Ba/Sr is 0.1, Ba/Rb is 8. Strontium isotope data show the only marked variance from ORB chemistry with LBB values ranging from 87Sr/ 86Sr=0.7020 to 0.7051. The low Sr abundances in the samples suggest the possibility of crustal Sr contamination to explain the radiogenic Sr enrichment. An alternate possibility is that the mantle source rocks were enriched in 87Sr. Variation within dredge hauls and between dredge sites may be explained by low-pressure fractional crystallization of magmas separated from the mantle at about 50 km depth. The basin probably began to open in middle to late Miocene time either by the disruption of a single andesitic island arc by splitting along its axis or by dilation of the area between two closely spaced concentric arcs. Mantle counterflow in the asthenosphere above the downgoing oceanic lithosphere slab is the probable driving force for dilation and has provided a continuous supply of parent material for the basalt of the basin floor.

Differentiation trends in the Warrumbungle Volcano, New South Wales, Australia

The volcanic rocks of the Warrumbungle Shield volcano in central western New South Wales, Australia show two distinct differentiation trends. One trend of differentiation extends from alkali basalts to phonolitic trachytes becoming increasingly undersaturated with silica and a second differentiation trend extends from trachybasalts to quartzbearing trachyte that becomes progressively more oversaturated. The differentiation trends are the product of low pressure fractional crystallisation of an alkali basalt in a high level magma chamber.

Tholeiitic andesite of high-pressure origin from the tweed shield volcano, northeastern New South Wales

A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab50An46Or4) and “megacrysts” of aluminian bronzite (Ca4Mg71Fe25) and relatively Ca-poor aluminian augite (Ca36Mg41Fe20). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca41Mg42Fe17) and olivine (Fa53). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma.