Petrologic Reconnaissance of Franciscan Metagraywackes from the Diablo Range, Central California Coast Ranges

The complexly folded and faulted Diablo antiform representsa core of pervasively metamorphosed Late Mesozoic Franciscanrocks surrounded by roughly contemporaneous, less deformed,only feebly recrystallized Great Valley strata. The contactbetween the two lithologic series is nearly everywhere a high-anglethrust, the Ortigalita fault. Thin sections of 679 metaclastics from the 3000 km² Franciscanarea of the Diablo Range have been studied. A very rough correlationseems to exist between the degree of textural reconstitutionand phase assemblage. Many feebly metamorphosed Franciscan rockscontain relict clastic biotite; chlorite, white mica, and minoramounts of pumpellyite appear to be newly generated from claysand detrital plagioclase which has been albitized. In more thoroughlyrecrystallized Franciscan rocks, lawsonite has grown by hydrationof the minor An component of plagioclase, but more commonlyby the inferred interaction of interstitial clays+calcium carbonate;some of these rocks carry metamorphic aragonite. Intensely recrystallizedFranciscan metagraywackes contain jadeitic pyroxene±glaucophane.The observed changes in mineral assemblages are thought to reflectprogressive pressure increment at nearly constant temperature. In the north-western portion of the Range, jadeitic metaclasticrocks are located along the faulted margin of the antiform.Elsewhere there appears to be no clear relationship betweensystematic Franciscan parageneses and the tectonic contact withrocks of the Great Valley sequence. Evidently Diablo Range metaclasticassemblages do not owe their present areal distribution to apostulated process involving tectonic overpressures accompanyingthrusting of the Great Valley strata over the Franciscan alongthe Ortigalita fault. Jadeitic pyroxenebearing metagraywackesalso appear to be unrelated to the emplacement of alpine-typeperidotites.     

Oxygen,hydrogen and carbon isotope studies of the franciscan formation,Coast Ranges,California

Analyses of 230 Franciscan rock and mineral samples, including the San Luis Obispo ophiolite, show that metamorphism produces no change in the δ¹⁸O of the graywackes (+11 to +14), but that igneous rocks become enriched in ¹⁸O by 2–6% and the cherts depleted by 5–10%. The shales are of two types, a high-¹⁸O type (+16 to +20) associated with chert and a low-¹⁸O type isotopically and mineralogically similar to the graywackes. The vein quartz (δ = + 15 to + 20) is invariably richer in ¹⁸O than the host rock quartz and in most of the rocks the δ¹⁸O of the clastic quartz is similar to the δ¹⁸O of the whole rock. Mineral assemblages are typically not in isotopic equilibrium. Although the δ¹⁸O values are very uniform (+13 to +16). the δ¹³C of vein aragonite and calcite is widely variable (0 to ? 14), implying that a major source of the carbon is oxidized organic material. The δD values of 83 igneous and sedimentary rocks are -45 to -80, exceptions are the Fe-rich minerals howieite and deerite, which have δD = ?100. All of these samples could have equilibrated with H₂O having δD ≈ +10 to ?20 and δ¹⁸O ≈ ?3 to +8. assuming temperatures of 100–300°C. However, the serpentines (δD ≈ ?85 to ?110) and the vein minerals (δD = ?23 to ?55) are exceptions. The vein minerals are 10–20%, richer in deuterium than the adjacent wall rocks; they formed from a relatively D-rich metamorphic water, typically at lower temperatures than did their host rocks. The isotopic compositions of the other Franciscan rocks were affected by three distinct events: (1) hydrothermal alteration of the ophiolite complexes and volcanic rocks as a result of submarine igneous activity at a spreading center or in an island-arc environment; (2) low-temperature, high-pressure regional metamorphism and diagenesis; and (3) a late-stage, very low temperature (<100°C) alteration of the ultramafic bodies by meteoric ground waters, producing lizardite-chrysotile serpentine. In the first two cases, the pore fluid involved in the alteration of the Franciscan rocks was sea water. However, this water became somewhat depleted in D and enriched in ¹⁸O during blueschist metamorphism, evolving to values of δD ≈ ? 20 and δ¹⁸O ≈ + 6 to + 8 at the highest grades. Except for one graywacke sample, the meteoric waters that affected the serpentinites did not significantly change the $\text{D}\text{H}$ ratios of the OH-bearing minerals in any other Franciscan rock.The δ¹⁸O values of orogenic andesites are too low for such magmas to have formed by direct partial melting of Franciscan-type materials in a subduction zone. Andesites either form in some other fashion, or the melts must undergo thorough isotopic exchange with the upper mantle. The great Cordilleran granodiorite-tonalite batholiths, however, are much richer in ¹⁸O and may well have formed by large-scale melting or assimilation of Franciscan-type rocks. The range of δD values of Franciscantype rocks is identical to the ?50 to ?80 range shown by most igneous rocks. This suggests that ‘primary magmatic H₂O’ throughout the world may be derived mainly by partial melting of Franciscantype materials, or by dehydration of such rocks in the deeper parts of a Benioff zone.     

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone,Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.     

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex(SFVC) comprise three distinct petrological groups based ontheir whole-rock geochemistry: (1) oceanic tholeiites; (2) transitionalalkali basalts and glasses; (3) high-Al, low-Ti tholeiites.Major and trace element, and Sr–Nd–Pb isotopic dataindicate that the oceanic tholeiites formed as low-degree partialmelts of normal mid-ocean ridge basalt (N-MORB)-source asthenospheresimilar in isotope composition to the East Pacific Rise today;the alkalic lavas were derived from an enriched source similarto that of E-MORB. The high-Al, low-Ti lavas resemble second-stagemelts of a depleted MORB-source asthenosphere that formed bymelting spinel lherzolite at low pressures. Trace element systematicsof the high-Al, low-Ti basalts show the influence of an enrichedcomponent, which overprints generally depleted trace elementcharacteristics. Tectonic discrimination diagrams show thatthe oceanic tholeiite and alkali suites are similar to present-daybasalts generated at mid-oceanic ridges. The high-Al, low-Tisuite resembles primitive arc basalts with an enriched, alkalibasalt-like overprint. Isotopic data show the influence of recycledcomponents in all three suites. The SFVC was constructed ona substrate of normal Coast Range ophiolite in an extensionalforearc setting. The close juxtaposition of the MORB-like olivinetholeiites with alkali and high-Al, low-Ti basalts suggestsderivation from a hybrid mantle source region that includedMORB-source asthenosphere, enriched oceanic asthenosphere, andthe depleted supra-subduction zone mantle wedge. We proposethat the SFVC formed in response to collision of a mid-oceanridge spreading center with the Coast Range ophiolite subductionzone. Formation of a slab window beneath the forearc duringcollision allowed the influx of ridge-derived magmas or themantle source of these magmas. Continued melting of the previouslydepleted mantle wedge above the now defunct subduction zoneproduced strongly depleted high-Al, low-Ti basalts that werepartially fertilized with enriched, alkali basalt-type meltsand slab-derived fluids. KEY WORDS: CRO; oceanic basalts; California     

Geomorphic controls on mercury accumulation in soils from a historically mined watershed, Central California Coast Range, USA

Historic Hg mining in the Cache Creek watershed in the Central California Coast Range has contributed to the downstream transport of Hg to the San Francisco Bay-Delta. Different aspects of Hg mobilization in soils, including pedogenesis, fluvial redistribution of sediment, volatilization and eolian transport were considered. The greatest soil concentrations (>30 mg Hg kg⁻¹) in Cache Creek are associated with mineralized serpentinite, the host rock for Hg deposits. Upland soils with non-mineralized serpentine and sedimentary parent material also had elevated concentrations (0.9–3.7 mg Hg kg⁻¹) relative to the average concentration in the region and throughout the conterminous United States (0.06 mg kg⁻¹). Erosion of soil and destabilized rock and mobilization of tailings and calcines into surrounding streams have contributed to Hg-rich alluvial soil forming in wetlands and floodplains. The concentration of Hg in floodplain sediment shows sediment dispersion from low-order catchments (5.6–9.6 mg Hg kg⁻¹ in Sulphur Creek; 0.5–61 mg Hg kg⁻¹ in Davis Creek) to Cache Creek (0.1–0.4 mg Hg kg⁻¹). These sediments, deposited onto the floodplain during high-flow storm events, yield elevated Hg concentrations (0.2–55 mg Hg kg⁻¹) in alluvial soils in upland watersheds. Alluvial soils within the Cache Creek watershed accumulate Hg from upstream mining areas, with concentrations between 0.06 and 0.22 mg Hg kg⁻¹ measured in soils 90 km downstream from Hg mining areas. Alluvial soils have accumulated Hg released through historic mining activities, remobilizing this Hg to streams as the soils erode.     

Late Quaternary landscape development along the Rancho Marino coastal range front (south‐central Pacific Coast Ranges,California, USA)

Late Quaternary landscape development along the Rancho Marino coastal range front in the central‐southern Pacific Coast Ranges of California has been documented using field mapping, surveying, sedimentary facies analysis and a luminescence age determination. Late Quaternary sediments along the base of the range front form a single composite marine terrace buried by alluvial fans. Marine terrace sediments overlie two palaeoshore platforms at 5 m and 0 m altitude. Correlation with the nearby Cayucos and San Simeon sites links platform and marine terrace development to the 125 ka and 105 ka sea‐level highstands. Uplift rate estimates based on the 125 ka shoreline angle are 0.01–0.09 m ka^?1 (mean 0.04 m ka^?1), and suggest an increase in regional uplift along the coast towards the NW where the San Simeon fault zone intersects the coastline. Furthermore, such low rates suggest that pre‐125 ka uplift was responsible for most of the relief generation at Rancho Marino. The coastal range front landscape development is, thus, primarily controlled by post 125 ka climatic and sea‐level changes. Post 125 ka sea‐level lowering expanded the range front piedmont area to a width of 7.5 km by the 18 ka Last Glacial Maximum lowstand. This sea‐level lowering created space for alluvial fan building along the range front. A 45 ± 3 ka optically stimulated luminescence (OSL) age provides a basal age for alluvial fan building or marks the time by which distal alluvial fan sedimentation has reached 300 m from the range front slope. Fan sedimentation is related to climatic change, with increased sediment supply to the range front occurring during (1) glacial period cold stage maxima and/or (2) the Late Pleistocene–Holocene transition, when respective increases in precipitation and/or storminess resulted in hillslope erosion. Sea‐level rise after the 18 ka lowstand resulted in range front erosion, with elevated localised erosion linked to the higher relief and steeper slopes in the SE. This study demonstrates that late Quaternary coastal range front landscape development is driven by interplay of tectonics, climatic and sea‐level change. In areas of low tectonic activity, climatic and sea‐level changes dominate coastal landscape development. When the sea‐level controlled shoreline is in close proximity to the coastal range front, localised patterns of sedimentation and erosion are passively influenced by the pre‐125 ka topography. Copyright © 2008 John Wiley & Sons, Ltd.     

Geochronological, isotopic, and geochemical data from Permo-Triassic granitic gneisses and granitoids of the Colombian Central Andes

New U–Pb SHRIMP ages in zircon, Ar–Ar ages in micas and amphiboles, Nd–Sr isotopes, and major and REE geochemical analyses in granitic gneisses and granitic stocks of the Central Cordillera of Colombia indicate the presence of a collisional orogeny in Permo-Triassic times in the Northern Andes related to the construction of the Pangea supercontinent. The collision is recorded by metamorphic U–Pb SHRIMP ages in inherited zircons around 280 Ma and magmatic U–Pb SHRIMP ages in neoformed zircons around 250 Ma within syntectonic crustal granitic gneisses. Magmatic U–Pb SHRIMP and Ar–Ar Triassic ages around 228 Ma in granitic stocks indicate the presence of late tectonic magmatism related to orogenic collapse and the beginning of the breakup of the supercontinent. During this period, the Central Cordillera of Colombia would have been located between the southern United States and northern Venezuela, in the leading edge of the Gondwana supercontinent.     

The relationship between the McLaughlin gold–mercury deposit and active hydrothermal systems in the Geysers–Clear Lake area, northern Coast Ranges, California

The Geysers–Clear Lake area has a long history of research on its active hydrothermal systems. It is a unique area containing a number of hydrothermal systems which include: the Geysers steam field, one of the largest vapor-dominated geothermal systems yet recognized; the McLaughlin gold deposit, an extremely well preserved hot-spring style gold deposit; and the Sulphur Bank mercury deposit, one of the first locations where geothermal systems were recognized as modern analogues to epithermal deposits. There is also a variety of active hot- and mineral-springs, including Wilbur Springs, or the Sulphur Creek district, which has been considered one of the type localities for connate fluids.The McLaughlin gold–mercury deposit is a fossil hot-spring system dominated by meteoric waters that exchanged with sedimentary rocks of the Great Valley sequence. Mineralization was syntectonic, occurring contemporaneously with fault movement. The fluids circulated in syntectonic dilation zones that resulted in, and maintained, high permeability of the fluid conduits permitting large volumes of fluid flow. The fluids precipitated metals in response to physical and chemical changes associated with boiling. The hydrothermal fluids that formed the McLaughlin deposit have the highest reservoir temperature, salinity and are isotopically the most enriched, of the Coast Range hydrothermal systems. The McLaughlin deposit is considered an end-member “fluid-dominated” hydrothermal system.The Geysers steam field, in its earliest phase was likely similar to the McLaughlin deposit being fluid-dominated and forming, at least on a small scale, a vein system enriched in silver and anomalous in gold, base metals, antimony and mercury. The hydrothermal system evolved into a vapor-dominated system as a result of decreased permeability of the reservoir, decreased recharge and/or increased heat flow. The modern day reservoir is encapsulated in impermeable rocks and is a “vapor-dominated” end-member hydrothermal system.Active hot- and mineral-springs in the Coast Ranges of northern California are intermediate between the fluid- and vapor-dominated end-member systems. The chemical and isotopic compositions of these fluids are the result of thermal processes and are not explained by simple mixing models between connate fluids and meteoric groundwater. Their isotopic and chemical composition is best explained by meteoric-dominated systems with repeated non-equilibrium subsurface vapor loss (evaporation) in a near closed system, with the relative deuterium and ¹⁸O enrichment proportional to the reservoir temperature.     

Chemically Distinct Mafic Dike/Sill Sequences of Contrasting Age Ranges, Sawyers Bar Area, Central Klamath Mountains, Northern California

Mafic hypabyssal rocks in the western Triassic and Paleozoicbelt provide important clues to the nature of accretion andarc evolution along this sector of the North American margin.In the east-central part of the belt, near Sawyers Bar, somediabases have been metamorphosed before and accompanying emplacementof the mid-Jurassic English Peak and Russian Peak granitoidswithin the North Fork/Salmon River + Stuart Fork amalgamatedterrane. Certain other dikes/sills, chiefly mafic microdiorites,cut the calc-alkaline plutons but are themselves deutericallyaltered; at least two of these mafic microdiorites near theEnglish Peak body possess hornfelsic textures. Thus, althoughmost mafic microdioritic hypabyssals seem to have been injectedafter granitoid emplacement, a few must have preceded plutonicintrusion. Macroscopic appearances, phase assemblages, mineralcompositions, and textures of the mafic microdioritic and metadiabasicdikes/sills are sufficiently alike to preclude the ready fieldand petrographic distinction of the different magma series.Bulk-rock chemistries fall into two groups, however, with slightlymore porphyritic, altered, synplutonic mafic microdiorite samplesbeing distinctly richer in Si, K, P, Rb, Sr, Zr, and light rareearth elements (LREE) relative to the Mg + Cr + Ni-rich, preplutonicmetadiabases. Analyzed mafic microdiorites have bulk-rock chemicaland isotopic compositions similar to the more ferromagnesianportions of the mid-Jurassic English Peak and Russian Peak plutoniccomplexes, whereas the metadiabases are comparable with theearly Mesozoic Salmon River metabasalts. Although the two groupsof dikes/sills probably overlap in age of emplacement, the maficmicrodiorite group is predominantly younger and uniform in oxygenisotopic composition (bulk-rock ¹⁸O 11•37, 11•4 and11•46) compared with the older, more intensely metamorphosed,and variably metasomatized Salmon River metadiabases (bulk-rock5¹⁸ 9•4, 11•0, and 15•3). Both types of maficdike/sill locally intrude the more easterly Stuart Fork terrane.Therefore, suturing and regional metamorphism of the outboardNorth Fork/Salmon River oceanic-island arc and inboard StuartFork subduction complex must have occurred during terminal stagesof injection of the pregranitoid diabases into the North Fork(oceanic-island basalts)/Salmon River (island-arc tholeiites)arc + Stuart Fork terrane, but before invasion of the amalgamatedterrane assembly by the calc-alkaline plutons and most compositionallyrelated synplutonic mafic microdiorite dikes/sills. Becauseof their lateral continuation both north and south of the SawyersBar area, the North Fork/Salmon River igneous suite documentsthe construction of an oceanic arc of considerable lateral extentin the central Klamaths before terrane accretion. Suturing wasimmediately followed by the mid-Jurassic intrusion of calc-alkalineplutons + syngranitoid mafic microdioritic hypabyssals.     

南秦岭花岗岩锆石U-Pb定年及其地质意义

锆石Ｕ－Ｐｂ定年结果表明,南秦岭勉、略构造带以北迷坝、光头山和东江口等花岗岩体形成于三叠纪（２０６￣２２０Ｍａ）,与南秦岭勉－略构造带洋盆的闭合时代及大别山超高压变质时代基本一致显示了它们的形成与勉－略古生代洋盆闭合后及华南陆块与华北陆块碰撞之间的内在联系。它支持华南和华北两大陆块最终在印支期碰撞的观点。     

Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite,California

The Coast Range ophiolite (CRO) of California is one of the most extensive tracts of oceanic crust preserved in the North American Cordillera, but its origin has long remained controversial. We present here new data on mineral compositions in mantle peridotites that underlie crustal sections of the ophiolite, and show that these are dominantly refractory harzburgites related to high apparent melting in a supra-subduction zone (SSZ) setting. Abyssal peridotite (characterized by high-Al spinels and relatively high Ti, Na, Nd, Sm, Lu, and Hf in pyroxene) occurs at one location where it is associated with SSZ mantle peridotite and volcanic rocks with both oceanic and arc-like geochemistry. SSZ mantle peridotites (characterized by intermediate-Cr/Al or high-Cr spinels, and by extremely low Ti, Na, Nd, Sm, Lu, and Hf in pyroxenes) are associated with crustal sections containing arc-related volcanic rocks, including boninites. This convergence between conclusions based on crustal lithologies and their underlying mantle sections confirms previous proposals that link the CRO to SSZ processes, and seriously undermines hypotheses that invoke formation of the ophiolite at a mid-ocean ridge spreading center.     

Biotites from Granitic Rocks of the Central Sierra Nevada Batholith, California

Biotites from plutonic recks of the central Sierra Nevada andInyo Mountains, California, have been examined and characterizedby powder X-ray diffraction and optical and chemical methods. Compositions of the biotites define a trend in the compositionaltriangle Fe+3 Fe+2Mg. When related to the experimentally studiedternary system KFe₃+3AlSisO₁₂H-₁-KFe₃+2 AlSi₃O₁₀(OH)₂-KMg₃AlSi₃O₁₀(OH)₂and coupled with the estimated positions of biotite solid solutionsfor different oxygen buffers, the trend suggests that oxygenfugacities in magmas during biotite crystallization were slightlyhigher than those defined by the Ni-NiO buffer. The compositionaldata also suggest that magmas were ‘buffered’ withrespect to oxygen by oxides existing within the magmas themselves. Correlation between the Fe/(Fe+Mg) ratio, an inferred temperatureindicator, and other elements is generally poor, which suggeststhat factors other than temperature at the time of crystallizationexerted an important influence on compositions.     

Hornblendes from Granitic Rocks of the Central Sierra Nevada Batholith, California

Twenty samples of hornblendes from rocks of 14 plutonic unitsin the central Sierra Nevada and Inyo Mountains, California,have been studied in detail. Optical, density, single-crystaland powder X-ray diffraction, and major and minor element chemicaldata are reported. The compositions of the hornblendes show only limited correlationwith the chemistry of the rocks in which they occurred. Hornblendesfrom granitic rocks of the eastern Sierra Nevada and Inyo Mountainshave a wide range of tetrahedral aluminum content which is oftenas low as three-quarters of an atom per formula unit, whereashornblendes from younger granitic rocks elsewhere in the SierraNevada batholith contain more than one atom of tetrahedral aluminumper formula unit. Because an increase of aluminum in tetrahedralco-ordination is considered indicative of higher temperaturesof crystallization, the observed differences in the hornblendessuggest that older plutonic rocks of the batholith may havebeen metamorphosed regionally or may have been affected by widespreadhydrothermal action prior to consolidation of later graniticrocks.     

Economic Aspects of Red Sands from the Southern Coast of Somalia

Analysis of Quaternary sands from coastal Somalia indicates the potential for economically useful glass-sand components and heavy minerals. The sands occur in four distinct stratigraphic levels and form subparallel ridges. Four components are present in all sand specimens: calcium carbonate, quartz, feldspars, and green hornblende; mineral compositions vary along a sea- ward-landward progression. The red sands are characterized by silica dissolution-precipitation features and alteration to laterite products.     

内蒙古中部花岗质岩类年代学格架及该区构造岩浆演化探讨

内蒙古中部广泛出露花岗质岩类,这些花岗质岩类的时空分布及岩石组合类型的变化,反映了华北板块北缘与蒙古陆块碰撞拼合的进程.本文从花岗质岩类的角度对古亚洲洋在内蒙古中部地区的演化进行了探讨.古亚洲洋在该区的演化经历了十分复杂的过程,包括奥陶纪双向俯冲、志留纪拼贴/增生、泥盆纪拉张、二叠纪南部带俯冲和北部带拉张、并以晚古生代末至早中生代初发生的陆-陆碰撞为标志宣告该区洋盆演化的结束.     

Comparative geochemical,magnetic susceptibility,and fluid inclusion studies on the Paleoproterozoic Malanjkhand and Dongargarh granitoids,Central India and implications to metallogeny

The Malanjkhand granodiorite (MG) hosting economic copper mineralization and the hitherto barren Dongargarh granitoids (DG) have subtle differences in their petrographic and bulk geochemical features. The two plutons are contiguous and occur in the northern part of the Bhandara Craton in Central India with intervening volcanosedimentary sequence of the Dongargarh Supergroup amidst older gneisses. The Dongargarh granitoids studied in two smaller units have higher bulk magnetic susceptibility than the Cu-bearing MG; the majority of samples studied from the latter being ilmenite-series rocks. DG crystallized at higher pressures compared to MG. Plagioclase composition ranges from albite to high bytownite in MG, whereas its compositional range is restricted to high andesine in DG. However, both intrusions give identical temperature ranges estimated by binary feldspar thermometry. Biotite in MG shows higher Fe/Mg ratios, as well as a greater range of compositional variation, than that in DG. MG has a moderately fractionated rare earth element distribution pattern without any significant Eu anomaly, showing depletion in mid-range rare earth elements (REE) and no depletion in heavy REE. DG is characterized by a prominent negative Eu anomaly. Geochemical features indicate subtle differences in the nature of source rocks and/or melting processes responsible for the generation of the two granitoids. MG displays more consistent bulk chemical features and is possibly a result of crystallization from a homogeneous granodioritic melt. DG displays a greater diversity and possibly incorporated a significant felsic crustal component that contributed to the parent melt. A fluid inclusion study of quartz grains from the granitoids and barren quartz veins occurring in MG indicates identical low-temperature nature of the fluid in both cases. They differ from the fluid in the mineralized zone in MG in the absence of a high-temperature component and CO₂ in the fluid. Late-stage fluids in DG and associated barren quartz veins compare well with those from MG. The hydrothermal activity following the granite emplacement seems to have operated under identical temperature conditions, and the aqueous fluid at the two occurrences seems to have been broadly similar. In both cases, internal evolution of the exsolved fluid to low temperatures and moderate salinity are visualized. Based on the existing information, the lack of ore potential in DG may be attributed to the metal and volatile (water + halogens) deficient nature of the parental granitic melt.     

Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith,southern California

Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and typically represent fragments of mafic magmas, cumulates, restites, or country rocks. To develop criteria for identifying dark enclaves of non-magmatic origin, we investigated dark enclaves from a complete spectrum of light (carbonate- or feldspar-rich) to dark (amphibole-rich, biotite-rich, or composite) enclaves, reflecting progressive thermal and chemical equilibration with the host tonalite, the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have geochemical characteristics that overlap those of literature-compiled igneous dark enclaves. When compared with modelled igneous differentiation paths, metasedimentary enclaves can have anomalous CaO and K₂O contents for a given SiO₂, but other major-element systematics may not deviate noticeably from igneous differentiation trends. In addition, the fact that literature-compiled mafic enclaves trend towards high K₂O + CaO suggests that not all mafic enclaves are of igneous origin. In this work, we provide criteria for identifying enclaves of possible metasedimentary origin.     

Rb-Sr geochronology and petrogenesis of granitoids from the Chhotanagpur granite gneiss complex of Raikera-Kunkuri region,Central India

The Precambrian Chhotanagpur granite gneiss complex (CGGC) terrain covers more than 80,000 sq km area, and is dominated by granitoid gneisses and migmatites. Recent geochronological data indicate that the CGGC terrain has witnessed five tectonomagmatic thermal events at: (i) 2.5–2.4 Ga, (ii) 2.2–2.0 Ga, (iii) 1.6–1.4 Ga, (iv) 1.2–1.0 Ga, and (v) 0.9–0.8 Ga. Of these, the third and the fourth events are widespread. The whole-rock Rb-Sr isotopic analysis of twenty granite samples from the CGGC of Raikera-Kunkuri region, Jashpur district, Chhattisgarh, Central India, yields two distinct isochrons. The eleven samples of grey granites define an isochron age of 1005±51 Ma with moderate initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7047±0.0065, which corresponds to the fourth tectonomagmatic event. On the other hand, the nine samples of pink granites indicate younger isochron age of 815±47 Ma with a higher initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7539±0.0066 that matches with the fifth phase of the thermal event. The data suggest emplacement of large bodies of grey granite at ∼1005 Ma that evolved possibly from precursors of tonalitic-granodioritic composition. Furthermore, the younger age (∼815 Ma) suggests the age of metasomatism, involving isotopic resetting, that resulted in genesis of pink granite bodies of limited areal extent. By analogy, the age of metasomatism (∼815 Ma) may also be taken to represent the age of Y-mineralisation in the Raikera-Kunkuri region of the CGGC terrain.     

Geochemical studies of granitoids from Shyok tectonic zone of Khardung-Panamik section,Ladakh, India

The Shyok tectonic zone lies to the north of Ladakh magmatic arc or the Ladakh batholith in the Trans-Himalaya of Ladakh district, J & K. Investigations were carried out on the granitoids exposed along Leh-Siachan highway between Khardung and Panamik villages. The granitoid bodies under study are: Khardung granite (KG), Tirit granite (TG) and Panamik granite (PG) belonging to Ladakh batholith, Shyok ophiolitic mélange and Karakoram batholith respectively. Though the granitoids belong to different litho-tectonic units, yet they have subduction related geochemical characters typical of Andean-type granitoids. Re-melting of crustal rocks of volcanic arc affinity has played an important role for the origin of KG rocks which are more evolved, while the TG and PG rocks represent transitional tectonic environment from primitive to mature arc.     

Influence of soil aggregation on slope stability in the Oregon Coast Ranges

Two major cohesionless soil series of the central Coast Ranges of Oregon were examined for soil and hydrologic properties. Although derived from different parent material, the Bohannon and Klickitat series exhibited nearly identical values of soil and hydrologic properties. Aggregation in both soils was found to be the most important property, for it influences shear strength and subsurface water movement, prime components of slope stability. The unusually high angle of internal friction of both cohesionless soils was 40° to 41°. The angle of internal friction was affected significantly by the mode of wetting. Subsurface water movement was predominantly by unsaturated flow. The influence of soil aggregation on pore-size distribution and arrangement created conditions where unsaturated flow was an effective means for water dispersal in both soils during most observed storm events. Considering the large area covered by these two soil series, aggregation appears to be an important slope-stability property with regional significance.