Mafic Varieties Research Articles

Northward subduction of Bangong–Nujiang Tethys: Insight from Late Jurassic intrusive rocks from Bangong Tso in western Tibet

New zircon LA-ICPMS U–Pb age data, whole-rock major and trace elements, and zircon Hf isotopic data of intrusive rocks from Larelaxin and Caima plutons in the southern margin of the Western Qiangtang subterrane provide important evidence of northward subduction of the Bangong–Nujiang Tethyan Ocean lithosphere. Host granitoids including quartz diorites, granodiorites, and syenogranites and associated mafic varieties including gabbroic enclaves, dioritic dikes and enclaves, and quartz dioritic enclaves are investigated in this study. Five host granitoid samples are dated at 163–160 Ma, and one dioritic dike and three dioritic enclave samples are dated at 162–158 Ma, which indicates that these rocks were contemporaneous. The quartz diorites and granodiorites are normal calc-alkaline I-type granitoids. The syenogranites are characterized by high SiO2 (74–77 wt.%) and differentiation index (DI = 92–97) and marked depletion in Ba, Nb, Ta, Sr, P, Ti, and Eu, thus indicating that they are highly fractionated I-type granitoids. The host granitoids exhibit uniform zircon εHf(t) values (− 1.4 to + 1.9). Considering the current data and their negative whole-rock εNd(t) values (− 4.7 to − 3.5) reported in recent studies, these rocks can be interpreted to have resulted from the partial melting of ancient mafic lower crust with varying contributions from mantle-derived or juvenile crust-derived components. One gabbroic enclave sample exhibits high Al2O3 (18.3 wt.%), low MgO (3.3 wt.%), high TiO2 (1.4 wt.%), and high Nb (24 ppm) and is geochemically similar to high-alumina basalt (HAB) and high-Nb basalt (HNB). This indicates that the gabbroic enclave originated from the partial melting of mantle wedge peridotite that was metasomatized by slab melting and subsequently experienced significant fractional crystallization of olivine and clinopyroxene. The dioritic dike and enclave samples yield εHf(t) values (− 1.6 to + 3.1) similar to those of the host granitoids. They most likely originated from magma mixing between mantle-derived and crust-derived melts at or close to the Moho. The intrusive rocks with geochemical diversity of the Larelaxin and Caima plutons from Bangong Tso in western Tibet can be interpreted to have resulted from melting, assimilation, storage, and homogenization (a MASH process) above subduction zones, thus indicating that the Bangong–Nujiang Tethyan Ocean lithosphere was subducted northward beneath the Western Qiangtang subterrane during the Late Jurassic.

In situ differentiation and evolution of potassic syenites from Svidnya, Bulgaria

Potassic syenites from Svidnya, Bulgaria crop out as small isolated bodies as the primary for this intrusion liquid has basic to intermediate composition. The evolution in a closed magma chamber created plutonic rocks ranging from basic (melasyenite) to acid (granite) and from metaluminous to peralkaline. The most mafic varieties show cumulative textures typical for orthocumulates with cumulus phases clinopyroxene, biotite, apatite and potassium feldspar as gravitational settling is a viable process for separation of particles in the bottom parts of magma chamber. In the middle stratigraphic level of biggest body modal igneous layering with development of dark (clinopyroxene + amphibole) and light (potassium feldspar) laminas was observed. Oscillatory crystallization around eutectic point resulted in cyclic separation of mafic and felsic phases in repetitive layers. Fractionation of Ca- and Al-rich phases—clinopyroxene, biotie and potassium feldspar created peralkaline residual liquid strongly enriched in HFS elements.

Alkaline magmatism in the Amambay area, NE Paraguay: The Cerro Sarambí complex

The Early Cretaceous alkaline magmatism in the northeastern region of Paraguay (Amambay Province) is represented by stocks, plugs, dikes, and dike swarms emplaced into Carboniferous to Triassic-Jurassic sediments and Precambrian rocks. This magmatism is tectonically related to the Ponta Porã Arch, a NE-trending structural feature, and has the Cerro Sarambí and Cerro Chiriguelo carbonatite complexes as its most significant expressions. Other alkaline occurrences found in the area are the Cerro Guazú and the small bodies of Cerro Apuá, Arroyo Gasory, Cerro Jhú, Cerro Tayay, and Cerro Teyú. The alkaline rocks comprise ultramafic-mafic, syenitic, and carbonatitic petrographic associations in addition to lithologies of variable composition and texture occurring as dikes; fenites are described in both carbonatite complexes. Alkali feldspar and clinopyroxene, ranging from diopside to aegirine, are the most abundant minerals, with feldspathoids (nepheline, analcime), biotite, and subordinate Ti-rich garnet; minor constituents are Fe–Ti oxides and cancrinite as the main alteration product from nepheline. Chemically, the Amambay silicate rocks are potassic to highly potassic and have miaskitic affinity, with the non-cumulate intrusive types concentrated mainly in the saturated to undersaturated areas in silica syenitic fields. Fine-grained rocks are also of syenitic affiliation or represent more mafic varieties. The carbonatitic rocks consist dominantly of calciocarbonatites. Variation diagrams plotting major and trace elements vs. SiO2 concentration for the Cerro Sarambí rocks show positive correlations for Al2O3, K2O, and Rb, and negative ones for TiO2, MgO, Fe2O3, CaO, P2O5, and Sr, indicating that fractional crystallization played an important role in the formation of the complex. Incompatible elements normalized to primitive mantle display positive spikes for Rb, La, Pb, Sr, and Sm, and negative for Nb–Ta, P, and Ti, as these negative anomalies are considerably more pronounced in the carbonatites. Chondrite-normalized REE patterns point to the high concentration of these elements and to the strong LRE/HRE fractionation. The Amambay rocks are highly enriched in radiogenic Sr and have TDM model ages that vary from 1.6 to 1.1 Ga, suggesting a mantle source enriched in incompatible elements by metasomatic events in Paleo-Mesoproterozoic times. Data are consistent with the derivation of the Cerro Sarambí rocks from a parental magma of lamprophyric (minette) composition and suggest an origin by liquid immiscibility processes for the carbonatites.

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

AbstractNeogene to Quaternary volcanic/magmatic activity in the Carpathian-Pannonian Region (CPR) occurred between 21 and 0.1 Ma with a distinct migration in time from west to east. It shows a diverse compositional variation in response to a complex interplay of subduction with rollback, back-arc extension, collision, slab break-off, delamination, strike-slip tectonics and microplate rotations, as well as in response to further evolution of magmas in the crustal environment by processes of differentiation, crustal contamination, anatexis and magma mixing. Since most of the primary volcanic forms have been affected by erosion, especially in areas of post-volcanic uplift, based on the level of erosion we distinguish: (1) areas eroded to the basement level, where paleovolcanic reconstruction is not possible; (2) deeply eroded volcanic forms with secondary morphology and possible paleovolcanic reconstruction; (3) eroded volcanic forms with remnants of original morphology preserved; and (4) the least eroded volcanic forms with original morphology quite well preserved. The large variety of volcanic forms present in the area can be grouped in a) monogenetic volcanoes and b) polygenetic volcanoes and their subsurface/intrusive counterparts that belong to various rock series found in the CPR such as calc-alkaline magmatic rock-types (felsic, intermediate and mafic varieties) and alkalic types including K-alkalic, shoshonitic, ultrapotassic and Na-alkalic. The following volcanic/subvolcanic forms have been identified: (i) domes, shield volcanoes, effusive cones, pyroclastic cones, stratovolcanoes and calderas with associated intrusive bodies for intermediate and basic calclkaline volcanism; (ii) domes, calderas and ignimbrite/ash-flow fields for felsic calc-alkaline volcanism and (iii) dome flows, shield volcanoes, maars, tuffcone/tuff-rings, scoria-cones with or without related lava flow/field and their erosional or subsurface forms (necks/ plugs, dykes, shallow intrusions, diatreme, lava lake) for various types of K- and Na-alkalic and ultra-potassic magmatism. Finally, we provide a summary of the eruptive history and distribution of volcanic forms in the CPR using several sub-region schemes.

Review: the charnockite problem, a twenty first century perspective

Beginning of the twentieth century was marked by coinage of a new rock name, Charnockite, first described as a hypersthene-bearing granite from Southern India. Since then charnockites have been described from most of the conti-nents and mostly restricted to high-grade belts. Later half of the last century saw a lively debate over an igneous versus metamorphic origin. However, two factors acted as deterrents for the resolution of the debate. First, charnockites and associated rocks occur in a variety of different structural setting and display diverse field rela-tions, attesting to possible different mode of origin. Second and possibly more important is the lack of consensus on the nomenclature of charnockites and associated rocks and this is commonly linked with the metamorphic versus magmatic perspective. Scanning the literature of this period makes one believe that both metamorphic and magmatic hypotheses are valid, but applicable to different field setting only. Before critically evaluating individual cases, it is imperative that a uniform approach in nomenclature should be agreed upon. It is proposed that name charnockite be adopted for any quartzofeldspathic rock with orthopyroxene, irrespective of its mode of occurrence, struc-tural setting and mode of origin. The associated more mafic varieties, be better described as mafic granulite, rather than basic charnockite. For the patchy charnockites of east Gondwana (including parts of Peninsular India, Sri Lanka and Antarctica), metamorphic transformation from amphibolite facies gneiss, by two different mechanisms: CO2 ingress from deep level, and drop in fluid pressure, has been proposed. However, all such patchy occurrence is not amenable to explanation by metamorphic trans- formation. In some instances, migmatisation of older charnockitic rocks is evident. Also pro- gressive charnockitisation relating patchy char-nockite to banded variety could be argued against on two counts: grain-size relation and time-relation. Larger bodies or bands have been explained as magmatic, but in many instances, from geochemical consideration alone. The compositional variation, commonly encoun-tered in many high-grade belts, if not described in terms of field relation, may lead to wrong no-tions of magmatic differentiation of mantle-de- rived melts. Crustal melting of dry granulite fa-cies source rocks has been proposed from geochemical and isotopic data of charnockitic intrusions. This model proposes high-tempera-ture melting of previously dehydrated and dry granulite source rocks. However, tectonic per-turbation subsequent to granulite facies meta-morphism that might have been responsible for such high temperatures, is not well constrained in this model. Finally, with advent of high- pressure dehydration-melting experiments in the nineties, dehydration-melting of mafic to intermediate composition, syn-kinematic with granulite facies metamorphism has been pro-posed.

The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source

Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years.

MAGMATIC ANHYDRITE IN PLUTONIC ROCKS AT THE EL TENIENTE Cu-Mo DEPOSIT, CHILE, AND THE ROLE OF SULFUR- AND COPPER-RICH MAGMAS IN ITS FORMATION

Anhydrite occurs along with biotite, feldspars, quartz, Fe-oxides, and sulfides in fine-grained igneous rocks that form the small “Porphyry A” stock at El Teniente. These minerals crystallized together in the space generated by the intrusion of this stock, and they do not replace preexisting phases. Planar crystal boundaries between anhydrite and other phases, as well as the fact that plagioclase and biotite are fresh and unaltered, also suggest that anhydrite is an igneous phase, and not the product of secondary hydrothermal alteration. Plagioclase is unusually Na rich, ranging from oligoclase in mafic varieties of these rocks with >20 percent biotite, to albite in more felsic samples with 20 percent. These unusual anhydrite-bearing plutonic rocks, which contain up to >3 wt percent S and >0.5 wt percent Cu, are isotopically similar to all other igneous rocks related to the deposit. These rocks formed during a late Miocene period of regional compressive deformation with no coeval volcanic activity in the area of the deposit, which prevented devolitalization of the oxidized parent magma in the large productive magma chamber inferred to have underlain El Teniente at >4 km below the paleosurface. This allowed igneous fractionation, involving the recharge of mantle-derived mafic magmas into the base of the chamber as well as volatile transfer and concentration near its roof, to produce the Cu- and S-rich magmas that formed the anhydrite-bearing intrusive rocks.

Timing of magmatism in the Kola alkaline province and the translation of isotope dates into geological processes

The Early Cretaceous alkaline magmatism in the northeastern region of Paraguay (Amambay Province) is represented by stocks, plugs, dikes, and dike swarms emplaced into Carboniferous to Triassic-Jurassic sediments and Precambrian rocks. This magmatism is tectonically related to the Ponta Porã Arch, a NE-trending structural feature, and has the Cerro Sarambí and Cerro Chiriguelo carbonatite complexes as its most significant expressions. Other alkaline occurrences found in the area are the Cerro Guazú and the small bodies of Cerro Apuá, Arroyo Gasory, Cerro Jhú, Cerro Tayay, and Cerro Teyú. The alkaline rocks comprise ultramafic-mafic, syenitic, and carbonatitic petrographic associations in addition to lithologies of variable composition and texture occurring as dikes; fenites are described in both carbonatite complexes. Alkali feldspar and clinopyroxene, ranging from diopside to aegirine, are the most abundant minerals, with feldspathoids (nepheline, analcime), biotite, and subordinate Ti-rich garnet; minor constituents are Fe–Ti oxides and cancrinite as the main alteration product from nepheline. Chemically, the Amambay silicate rocks are potassic to highly potassic and have miaskitic affinity, with the non-cumulate intrusive types concentrated mainly in the saturated to undersaturated areas in silica syenitic fields. Fine-grained rocks are also of syenitic affiliation or represent more mafic varieties. The carbonatitic rocks consist dominantly of calciocarbonatites. Variation diagrams plotting major and trace elements vs. SiO2 concentration for the Cerro Sarambí rocks show positive correlations for Al2O3, K2O, and Rb, and negative ones for TiO2, MgO, Fe2O3, CaO, P2O5, and Sr, indicating that fractional crystallization played an important role in the formation of the complex. Incompatible elements normalized to primitive mantle display positive spikes for Rb, La, Pb, Sr, and Sm, and negative for Nb–Ta, P, and Ti, as these negative anomalies are considerably more pronounced in the carbonatites. Chondrite-normalized REE patterns point to the high concentration of these elements and to the strong LRE/HRE fractionation. The Amambay rocks are highly enriched in radiogenic Sr and have TDM model ages that vary from 1.6 to 1.1 Ga, suggesting a mantle source enriched in incompatible elements by metasomatic events in Paleo-Mesoproterozoic times. Data are consistent with the derivation of the Cerro Sarambí rocks from a parental magma of lamprophyric (minette) composition and suggest an origin by liquid immiscibility processes for the carbonatites.

The Western Belt granite of Peninsular Malaysia: some emergent problems on granite classification and its implication

The Peninsular Malaysian granites have been grouped into two granite provinces namely Western and Eastern Belt granite. The Western Belt granites have been considered as constituting an exclusively ‘S’ type granites. The ‘S’ type features in the granites are, (a) high initial87Sr/86Sr isotope ratio, >0.710, (b) low Na2O content, <3.2% Na2O in rocks with ∼5% K2O, (c) narrow range of felsic rock (SiO2: 65.95 to 77.4%), (d) high K2O/Na2O ratio, 1.4–2.8 (‘S’ type: 0.9–3.2), (e) usually ilmenite bearing and (f) contain pelitic or quartzose metasedimentary xenoliths. However, detailed study of published and unpublished field and geochemical report revealed that the Western Belt granite consists of mixed ‘I’ and ‘S’ type features and thus the batholiths cannot be designated as exclusively ‘S’ type. The ‘I’ type features are (a) Alrich mineral such as sillimanite and cordierite are absent, (b) occurrence of primary wedge sphene and pale green amphibole especially in the northern part of the batholith, (c) occurrence of pinkish K-feldspar crystal (usually as phenocrysts), (d) occurrence of mafic, hornblende bearing enclaves, (e) increasing ACNK values with SiO2, (f) increasing peraluminosity towards the most differentiated rocks which is in contrast to the ‘S’ type granite (increasing peraluminosity towards the most mafic varieties) and (g) showing a similar trend to the ‘I’ type granite in P2O5 vs. Rb and A-B plots. Implication of this study indicates that the Western Belt granite is not solely derived from metasediments. The study favours a mixed origin of crustal material such as metapelites, greywackes and metaigneous rocks.

Geochemistry of post-1540 Ma granites in the Cloncurry District, Northwest Queensland

Proterozoic Fe oxide Cu-Au-Co mineralization in the Cloncuriy district is spatially and temporally related to intrusion of voluminous postpeak metamorphic granites that form most of the Williams and Naraku batholiths. Detailed mapping and geochemical study of these granites in the southern part of the district have identified two groups of intrusions that are referred to here as the Cloncurry Supersuite and Eureka Supersuite.The Cloncurry Supersuite is composed mainly of monzogranite and syenogranite which consist of quartz, plagioclase, alkali feldspar, hornblende, biotite with accessory magnetite, titanite, apatite, and zircon. The timing of intrusion of the Wiley Igneous Complex is interpreted to be pre- or syn-D 3 , based on overprinting relationships, while hornblende geobarometry suggests an emplacement depth of approximately 14 km. The granites are subalkaline, high-K, metaluminous rocks and range from 59 to 74 wt percent SiO 2 . Increasing SiO 2 is accompanied by increasing K 2 O, Rb, and Th, and decreasing MgO, Fe 2 O 3 , FeO, TiO 2 , CaO, P 2 O 5 , Sr, Eu, Zr, Cu, Zn, Ni, V, and Sc. This is consistent with fractionation of observed phases including plagioclase, amphibole, biotite, magnetite, titanite, and apatite.The Eureka Supersuite ranges from diorite to syenogranite in composition. More mafic varieties contain pyroxene and hornblende, while more felsic varieties contain biotite as the only ferromagnesian silicate. Sodiccalcic alteration events are closely linked in space and time to intrusion of Eureka Supersuite magmas. The timing of intrusion is interpreted to be pre- and post-D 3 , based on overprinting relationships between different plutons and mylonitic fabrics. Hornblende geobarometry suggests an emplacement depth of approximately 10 km for the Mount Angelay Igneous Complex. Eureka Supersuite granites in the Mount Angelay Igneous Complex range from 51 to 76 wt percent SiO 2 . The granites are alkaline and K 2 O-rich, leading to their classification as a shoshonite series. With increasing SiO 2 content, TiO 2 , Al 2 O 3 , Fe 2 O 3 (T), MnO, MgO, CaO, P 2 O 5 , Sr, Eu, Cu, Zn, Ni, V, and Sc decrease, indicating fractionation of observed minerals including apatite, magnetite, pyroxene, hornblende, plagioclase, and biotite. Some samples are characterized by very high abundances of K 2 O, P 2 O 5 , Sr, Ba, and light rare earth elements (LREE) compared to other Eureka Supersuite samples. This may reflect their derivation from a source that was variably enriched in incompatible elements. Compared with many other shoshonitic rocks, high-K Eureka Supersuite samples are strongly depleted in Sr and to a lesser extent Eu, and enriched in Y and heavy rare earth elements (HREE). This suggests that garnet was not a stable phase in the source region, that the source contained plagioclase as a stable phase, and/or the granites evolved by extensive plagioclase fractionation.Partial melting in the lower crust to generate post-1540 Ma granites was most likely caused by intrusion of basic magma into the crust. Eureka Supersuite magmas were probably derived from partial melting of variably incompatible element-enriched gabbroic rocks, while Cloncurry Supersuite magmas were most likely derived from partial melting of more felsic material of dioritic to tonalitic composition.

A re-examination of the typology of peraluminous granite types in intracontinental orogenic belts

AbstractConventional rock classification diagrams do not distinguish the variety of peraluminous rock series. Moreover, peraluminous granite types have not been clearly discriminated in recent revisions. The study of several peraluminous series in different intracontinental orogenic belts reveals that four distinct groups can be defined. Using an A-B diagram, these four groups are: (1) highly peraluminous granitoids (hP) characterised by high A values and typified by an increase in peraluminosity toward the most mafic varieties; (2) moderately peraluminous granitoids (mP) which occupy the intermediate field and generally show increasing peraluminosity towards the most felsic varieties; (3) low peraluminous granitoids (IP) which plot in the lowest part of the peraluminous field defining negative slope trends; (4) highly felsic peraluminous granites (fP) with poorly defined variation trends.In intracontinental orogenic belts, the genesis of peraluminous granitic series is favoured by the abundance of fertile crustal protoliths, mainly metapelites, metaigneous rocks and metagreywackes. The difficulty of attaining temperatures in excess of 950°C at lower crustal levels during the tectonothermal evolution of thickened crust, inhibits the partial melting of more basic sources. Although the physical parameters of the melting process influence their chemical and mineralogical characteristics, source rock composition ultimately determines the degree of peraluminosity of the granitic series.

An enriched mantle source for potassic basanites: evidence from Karisimbi volcano, Virunga volcanic province, Rwanda

Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K2O/Na2O≥1, and enriched in incompatible elements, with chondrite normalised (La/Yb)n>18 and Nb/Zr>0.25. The 87Sr/86Sr and 143Nd/144Nd ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances >4% there is no systematic variation of isotope ratios with fractionation. At >4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a “primitive” K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both endmembers are mantle-derived and isotopically distinct, with the PKB being characterised by 87Sr/86Sr up to 0.707 and 143Nd/144Nd as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties 206Pb/204Pb remains roughly constant at ∼19.2 while 207Pb/204Pb and 208Pb/204Pb vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at ∼1 Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated 208Pb/204Pb and 208Pb*/206Pb* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated 207Pb/204Pb ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (∼27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at ∼1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was ≥5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high 208Pb/204Pb and 208Pb*/206Pb* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.

Thermal alteration and rank variation of coals in the Matanuska field, south-central Alaska

The Matanuska Valley of south-central Alaska is a geologically unique region that shows a variety of contact-metamorphic phenomena resulting from the thermal metamorphism of coals by igneous intrusives. The coals of the Paleocene Chickaloon Formation vary widely in rank from subbituminous to meta-anthracite. Coals of the lower or western Matanuska Valley are predominantly high-volatile bituminous, coals of the central Matanuska Valley are predominantly medium- to low-volatile bituminous, and coals of the upper or eastern Matanuska Valley are predominantly anthracite. The Matanuska Valley contains relatively small (approximately 135 million metric tons) but important resources of bituminous coal and minor quantities of anthracite. Structural deformation in the Matanuska Valley increases from west to east as does the size and frequency of igneous intrusive masses. The intrusives represent one general period of Pliocene volcanic activity that antedates the tectonic disturbance that caused the maximum deformation of the coal-bearing rocks. The intrusive rocks vary in composition from felsic to mafic varieties and in size from stocks to small dikes and sills. Although locally the intrusives have caused beneficial improvement in rank, they have generally introduced factors that have made an undetermined percentage of the coal areas of the central and eastern Matanuska Valley of doubtful commercial value. The thermal effects of contact metamorphism have produced a large amount of natural coke along with some anthracite and graphite. Petrologic compositions of Matanuska Valley coals show that vitrinite contents increase with rank, whereas liptinite and inertinite contents generally decrease with rank. The sample analyses show an overall range in vitrinite reflectance from 0.47% to 5.34%. Moist, mineral-matter-free heating contents range from 12, 322 to 14, 890 Btu lb −1. Chemical compositions of coal ashes show that Fe 2O 3, CaO, P 2MgO, SO 3, BaO, SrO, and Mr 3O 4 contents increase with rank, whereas K 2O and SiO 2 contents decrease with rank.

Petrographical characteristics of the koperberg suite, South Africa — an analogy to massif-type anorthosites?

A swarm of anorthositic/dioritic intrusives, collectively termed the Koperberg Suite, is scattered over an area of about 3000 km2 in the northwestern Cape Province (Namaqualand), South Africa. The intrusive suite is renowned for its associated magmatic copper sulphide deposits. Transgressive field relationships and the preservation of primary igneous features suggest emplacement of the Suite after the main metamorphic event, which has been dated at c. 1200 Ma. The distribution of the intrusions seems to have been controlled by vertical shearing which is manifested by antiformal disruptions (‘steep structures’) in the country rocks. Post-intrusion movement along some of these structures is deemed responsible for metamorphic textures in some mafic bodies. Igneous breccias associated with some intrusions present evidence of explosive tectonics at the time of emplacement. The rock types which compose the mafic suite include andesine-anorthosite, leucodiorite, hypersthene- and mica-diorite, hypersthenite and glimmerite. The occurrence of composite intrusions with two or more mafic varieties, and the modal gradation between the rock types suggest consanguinity. The anorthosite appears to be the oldest member of the series, followed by increasingly more mafic varieties such as leucodiorite, hypersthene-diorite (norite) and hypersthenite. Owing to the presence of virtually monomineralic rock types, various differentiation trends are recognized. Series that are dominated by hypersthene, brown mica, iron-titanium oxides and amphibole can thus be delineated. Microprobe analyses of the major silicates indicate plagioclase compositions of andesine, En58–68 for hypersthene, and brown mica compositions on the biotite/phlogopite boundary. The predominance of anorthosite and leucodiorite in the suite suggest an affinity with the massif-type anorthosite kindred. This relation is supported by other criteria such as the preponderance of hypersthene, the age of emplacement, the sequence of intrusion, En/An-ratios of plagioclase and hypersthene, and the association of syenitic rocks as well as iron-titanium oxide- and apatite-rich assemblages. It is provisionally proposed that the Okiep District represents the lower levels of an anorthosite complex of the massif-type.

Plagiogranite and keratophyre in ophiolite on Fidalgo Island, Washington

A sequence of Jurassic rocks on Fidalgo Island, Washington, is interpreted to be ophiolite. The order of rock types, from the base upward, is serpentinite, layered gabbro, a dike complex made up mostly of plagiogranite, volcanic rocks that are dominantly keratophyre, coarse breccia with clasts of keratophyre and plagiogranite, pelagic argillite, and siltstone-sandstone turbidites. The plagiogranites and keratophyres have identical chemical compositions and are mutually gradational in field setting and textures, all of which suggests that they are cogenetic. These rocks are distinguished from calc-alkalic rock types by their very low content of K 2 O (where SiO 2 = 70%, K 2 O = 0.2% to 0.7%). Metasomatic alteration of the rocks appears to be insignificant, judging from (1) well-preserved primary igneous textures, (2) well-preserved primary intrusive and extrusive contacts, and (3) uniformity of chemical composition across igneous units. An oceanic origin of the ophiolite is suggested by the capping of pelagic sediments. Their fine grain size, abundance of radiolaria, and enrichment in Mn and other metals are virtually identical to those of modern Pacific pelagic sediments and unlike that of arc or epicontinental sediments. This interpretation conflicts with the apparent paucity of plagiogranite and keratophyre on the present-day sea floor. Field relations and chemical trends indicate that the plagiogranite-keratophyre magma is not the product of fractionation of the same melt that crystallized layered gabbro. High water content of the plagiogranite-keratophyre magma is indicated by hydrothermal alteration of the gabbro near plagiogranite intrusions and the occurrence of hornblende instead of pyroxene in mafic varieties. We suggest that this water is from the sea and that the anomalously low K 2 O content of these magmas is due to exchange with sea water.

Potassium-argon dating of volcanic rocks

The potassium-argon method of geochronometry may be applied quantitatively to biotite, hornblende and sanidine if these minerals have been kept at low temperature since their formation. Fine-grained whole rocks, particularly of mafic varieties, may also have 100 % retentivity under suitable conditions. Other whole-rock samples may give 75 – 85 % of the true age. Young volcanic glasses may also be used for dating if they have not been devitrified but probably give somewhat low ages even in the best cases.