The Transition to Potassic Alkaline Volcanism in Island Arcs: The Ringgit--Beser Complex, East Java, Indonesia

The origin of potassic lavas with within-plate characteristicsin island are settings is unclear. The volcanic complex of Ringgit—Beser,situated in eastern Java, has erupted lavas of both normal islandare calc-alkaline type and atypical potassic lavas, includingsome highly magnesian lavas. The occurrence of these primitivelavas gives an unusual insight into the source characteristicsof the potassic lavas. The lavas from Ringgit—Beser have a wide range of K₂O(1.1–6.4 wt. %) and MgO contents (18.0–1.6 wt.%).The most magnesian lavas have high Ni and Cr contents. The calc-alkalinelavas have incompatible trace element patterns typical of islandare lavas with enrichments in large ion lithophile elements(LILE) and light rare earth elements (LREE) relative to highfield strength elements (HFSE) and heavy REE (HREE). The potassiclavas may be divided into two series on the basis of Ba andNb contents, with the enriched potassic (EK) series having higherBa and Nb contents for a given MgO content than the potassic(K) series. The EK and K series lavas have some incompatibletrace element ratios similar to within-plate lavas (e.g., highCe/Pb, low LILE/HFSE ratios, and low B/Be). However, both theEK series and K series lavas have negative Ti and Zr anomalies,and the EK series lavas have high Ba/La similar to are lavas.There is little distinction in Sr and Nd isotopes between theK and EK series, but the calc-alkaline lavas have lower ₈₇Sr/₈₆Srand higher ₁₄₃Nd/₁₄₄Nd ratios than the potassic lavas. The EKseries lavas have lower ₂₀₆Pb/₂₀₄Pb and higher ₂₀₈Pb/₂₀₄Pb thanthe K series lavas, but similar ₂₀₇Pb/₂₀₄Pb ratios. The K serieslavas define an almost horizontal trend in ₂₀₇Pb–₂₀₆Pbspace. The Pb isotopic ratios indicate that the EK series lavasare derived from a single mantle source, whereas the K seriesoriginate from a mixture of two mantle components. Calc-alkalinelavas have Pb isotope ratios similar to other calc-alkalineand tholeiitic lavas from Java, and plot on a mixing line betweenIndian Ocean mid-ocean ridge basalt (MORB) and Indian Oceansediment. Incompatible trace element and Pb isotope data for the calc-alkalinelavas indicate that these lavas have a similar source to othercalc-alkaline lavas erupted in Java, namely melts of the IndianOcean MORB mantle fluxed by fluids from the subducted slab.The potassic lavas originate from enriched mantle sources withinthe wedge which have not been affected by recent subductionprocesses. The EK series lavas are derived from a metasomatizedzone which has EMI-type characteristics. The K series lavasare derived from mixing of melts from Christmas Island-type(EMII) mantle and the metasomatized zone. The metasomatizedzone is probably situated at the base of the lithosphere andthe Indian Ocean MORB and Christmas Island-type mantle componentsare situated in the asthenosphere of the wedge. Isotopic datafor Ringgit—Beser lavas confirm that the mantle wedgeof the Sunda arc is extremely heterogeneous (Foden & Varne,1980; Varne, 1985; Wheller et al., 1987). The similarity in geochemistry between Indonesian potassic lavasand those erupted in continental settings indicates that themagma source is essentially the same, namely a metasomatizedphlogopite-rich layer generated by melts of recycled subductedlithosphere. The lack of negative Ti anomalies in the continentalpotassic lavas is ascribed to lower oxidation states in themantle in continental settings.     

Stratigraphy and Petrology of the Mulcahy Lake Layered Gabbro: An Archean Intrusion in the Wabigoon Subprovince, Ontario

The Mulcahy Lake gabbro is an Archean layered intrusion of tholeiiticbulk composition located in the Wabigoon subprovince. The intrusionis 6 km thick at the thickest and is exposed over an area of63 km². It intrudes basaltic to siliceous volcanics of the CrowLake-Savant Lake greenstone belt and is intruded by the Atikwabatholith. Zircon U-Pb data indicate crystallization at 27322+1·0/–0·9m·y. Principal phases are plagioclase, orthopyroxene, augite (andpigeonite in iron-rich rocks), olivine, hornblende and magnetite.Olivine is confined to several horizons. Apatite and then zirconare prominent accessory phases at advanced stages of fractionation.Plagioclase, pyroxenes and olivine are cumulate phases. Hornblendeis invariably an intercumulus phase. Magnetite is ubiquitousthroughout the intrusion, generally as a cumulate phase, andforms centimeter thick layers in fractionated rocks. Fractionationfollowed a tholeiitic trend with iron enrichment in the liquid. The intrusion is divided into lower, mixed, middle, upper andmarginal zones. The lower and middle zones are 2·0 and2·5 km thick respectively. The upper zone is approximately1 km thick, and the marginal zone is measured in hundreds ofmeters. A 200 m thick mixed zone is interposed between the lowerand middle zones. The base of the lower zone consists of ultramaficunits containing olivine of Fo₈₂. The top of the zone has olivineof Fo₂₈. Fractionation of the lower zone, from the floor up,was interrupted by the introduction of pristine liquid whichmixed with more dense and cooler residual liquid in the chamberto form the mixed zone. Further introduction of several minorpulses of liquid constructed the lower part of the middle zone.The upper part of the middle zone was constructed from a majorpulse of liquid plus several minor pulses each of which is representedby reversals in cryptic layering. The upper zone consists ofultramafic to iron-rich gabbro cumulates formed by cooling throughthe roof plus horizons formed by influx of pristine liquid.Marginal zone rocks represent cooling through the walls of theintrusion. Rhythmic layering is well developed in lower and middle zonecumulates. Petrofabric data show that orthopyroxene has a lineationin the plane of layering and parallel to structures suggestiveof flow. Plagioclase laths also have a preferred orientationin many cumulates and in unlayered gabbros as well. Flow, possiblylaminar, of liquid-crystal material is suggested and may belinked to the ultimate development of layering. Pressure during the course of crystallization probably was greaterthan 2 and less than 5 kb. Temperatures estimated from pyroxenesvaried from approximately 1200 to 1000 °C.f_o², is not wellconstrained but was sufficient to allow the formation of thinlocal magnetite cumulates late in the crystallization. The primarymelt was hydrous as indicated by the presence of hornblende.It is very unlikely that the melt was saturated with water duringcrystallization of the cumulate phases.     

Scanning electron microscopy of Pleistocene tills in Estonia

Tills from four Pleistocene glaciations were recovered from drill cores in Estonia and subjected to particle size and microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thick continental ice-sheets following the transport of, at most, several hundred kilometers during four Fennoscandian glaciations. The main problem is to determine if the type and range of microtextures present on the grain surfaces are diagnostic of transport in continental ice. The frequency of occurrence of microtextures including fractures, abrasion, and relief features are used to test the ability of continental ice to damage quartz particles emplaced as till. The range of quartz dissolution and presence of coatings on grains are also used to reconstruct the paleoenvironment that existed prior to transport as well as to estimate diagenetic effects that occurred following emplacement. The available data indicate a high degree of reworking of quartz grains from one glaciation to another. While the shapes and microtextures of grains from source rocks are not known, the great range of fracture and abrasion microfeatures, and high frequency of occurrence on grains in all tills, indicate that glaciers are effective crushing agents. An increase in the prevalence of chemically etched grains from older to younger tills suggests that some grains ( c . 50%) escape crushing, either because of preservation in the ice and lack of grain-to-grain contact, or as a result of massive reworking of weathered grains following interglaciations.     

RIPPLE MARK INDICES AND THEIR USES

The following dimensionless parameters (two of them well-known and five of them new) are defined for determination of ripple mark geometry: ripple index (RI), ripple symmetry index (RSI), continuity index (CI), bifurcation index (BI), straightness index (SI), and two different parallelism indices (PI₁ and PI₂). In general, RI = 15 or less indicates wave or water current origin; RI = 17 or more indicates wind or swash origin. RSI = 1.5 or less indicates wave or swash types; RSI = 3 or more indicates wind or water current types. CI = 15 or more suggests wind or wave origin; CI = 10 or less suggests water current origin. BI = 10 or more suggests wave varieties; BI = 1 or less suggests wind varieties. SI = 10² or more indicates wind or deep-water wave types; SI = 15 to 10² indicates wind or wave types; SI =4 or less indicates current types. PI1 = 7 or more suggests wave origin; PI1 = 1 or less suggests water current origin. PI2 = 0.4 or more is probably the result of swash or water current action. PI2 = 0.2 or less is probably the result of wind or wave action. Longitudinal ripple marks (such as rib-and-furrow) and deformed or modified varieties (such as flat-topped tidal-flat ripple marks and nearly- flat-topped intermittent creek ripple marks) have been excluded, inasmuch as (1)they are commonly easy to identify from their appearance, and (2)they are difficult to measure with ordinary methods. Plots of two indices against each other on coordinate paper can be particularly useful; the best combinations are RI vs. RSI, and RI vs. PI1, although several other pairs are almost as good. Where all seven parameters can be obtained, the confidence one can have in the interpretation is close to 98%. The effects of current bias, or depth bias, on wave-type ripple marks, extend to both the symmetry (RSI) and to sediment-transport interpretations. Unless the investigator is reasonably sure that no such bias is present (i.e., RSI = 1.0 instead of some significantly higher value such as 1.5), wave-type ripple marks cannot be used to determine direction of either wave approach or sediment transport. If no such bias is present, wave-type ripple marks still cannot be used to determine precise sediment transport direction. If RSI = 1.0 precisely, it is not even necessary that the ripple crests parallel the waves that formed them. The same restrictions apply to the interpretation of micro-crossbedding (that is, ripple mark internal structure). Despite these seemingly severe limitations, general geometry commonly permits a reliable interpretation, and hence ripple marks can provide a great deal of useful data for paleogeographic interpretations. The swash-zone variety of ripple marks includes two sub-types: those modified by a small but unmistakeable hydraulic jump, and those not so modified. The RI can be used to distinguish between these two, even when they were not observed to form.     

Geochemistry of till in perennially frozen terrain of the Canadian shield— application to prospecting

Shilts, W. W. 1977 06 01: Geochemistry of till in perennially frozen terrain of the Canadian shield - application to prospecting.
Geomorphological (drumlins, ribbed moraine) and geochemical features associated with till in the District of Keewatin are arranged in belts or dispersal trains paralleling the main direction of ice flow. Both types of features can be related to chemical and physical characteristics of specific types of source rocks.
For dispersal studies in perennially frozen terrain, till samples were collected from mudboils at spacings of approximately 1.6 km. The texture of till samples varies significantly from sample site to sample site because of varying source-rock lithologies and periglacial processes. Thus, because fractions coarser than clay are mostly quartz and feldspar and contain very little metal after weathering in the active layer, they were removed by centrifugation so as not to mask the 'true' relative metal contents of samples. The clay-sized fraction was separated from till samples and analyzed on the assumption that it contained scavenging phases that adsorb cations in proportion to those released by weathering of mineralized particles that were originally physically dispersed by glacial action.
Dispersal patterns of copper, zinc, nickel, and uranium were derived for approximately 1000 samples evenly distributed over a 2500 km² grid. From these maps, large dispersal trains of copper and nickel were found, and known areas of high potential for uranium and Cu-Zn mineralization were clearly indicated.     

The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H

Phase relations for the bulk compositions 3CaO·2FeO_x·3SiO₂+excessH₂O and CaO·FeO_x·2SiO₂+excess H₂O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO₂. Andradite, Ca₃Fe³⁺₂Si₃O₁₂, synthesized above 550 °C hasan average unit cell edge, a_o, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, a_o increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P_fluid andradite is stable above an f_O2of 10¹⁵ bar at 800 °C and 10-³² bar at 400 °C. At lowerf_O2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe²⁺SiO₄)+ wollastonite and kirschsteinite+xonotlite (Ca₆Si₆O₁₇(OH)₂). Synthetic hedenbergite, CaFe²⁺Si₂O₆, has average unit cell dimensionsof a_o = 9.857± 0.004 Å, b_o = 9.033±0.002Å, c_o = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P_fiuid, hedenbergite is stable below an f_O2 of10-¹³ bar at 800 °C and 10-²⁸ bar at 400 °C. Above thesef_O2 values, hedenbergite+O₂ breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the f_O2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H₂O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences.     

Zircon microstriators in the sand of auriferous Andean tills: fine tools and noble metals

Chemically inert and physically hard minerals, of which zircon is universally present and usually abundant, are minor but important components of glacial gold and tin placer deposits. Zircons and other much less common resistant minerals inflict major damage on light minerals, of which quartz is the dominant, chemically resistant member. Because of its sharpness inherited from a strong crystal morphology, and overall prismatic form, zircon is especially important as an abrasive mineral in glacial systems. Its chemically inert nature, its dominancc in terms of hardness over light minerals, and its abundance amongst other hard minerals makes it unique and important as a microstriator. Transported in a highly viscous glacial medium, it is capable of damaging other softer grains with aggressive crushing, chipping, striating, abrading and polishing processes. These occur in both coarse-grained gravelly sand and in fine-grained clayey silt matrices at the base of the icc. Zircon grains tend to serve many functions, initially as inclusion tools in larger feldspar grains and as 'studs' in quartzite grains. Wearing first on points, and later, following liberation, they assume a shape by honing, faceting and fracturing as tools and as grit that allows them to act as microstriators, inflicting damage on other particles in the basal ice. With a hardness of 7.5, lacking significant cleavage, and exhibiting strong crystal form. the finer-grained zircons appear to abrade and striate quartz (hardness 7.0). feldspars (hardness 6.0). garnets (hardness 6.5–7.5), and gold (hardness 2.5-3.0). A detailed study of Bolivian tills shows the dominant form of the zircon striator to be an elongate, pencil shape (euhedral polygonal prism with sharp, pyramidal terminations) that shows various degrees of abrasion, and ranges from wide grains with dull edges to narrow grains with sharp edges (typical pencil form).