Igneous Terrane Research Articles

Zircon geochronology of high-grade metamorphic rocks from outcrops along the Prince Olav Coast, East Antarctica: Implications for multi-thermal events and regional correlations

This paper reports laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ~ 990 Ma metamorphism of garnet–sillimanite–biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet–gedrite–biotite–chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600–580 Ma, 580–550 Ma, and 550–500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ~ 990–930 Ma without younger overprinting. Extensive high-grade metamorphism during ~ 650–500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580–560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.

The Pilot Knob iron ore deposits in southeast Missouri, USA: A high-to-low temperature magmatic-hydrothermal continuum

The Mesoproterozoic St. Francois Mountains igneous terrane in southeast Missouri, USA, contains eight major and several minor IOA/IOCG-type deposits. This study focuses on the Pilot Knob deposits, i.e., the largely massive Pilot Knob Magnetite (PKM) deposit and the Pilot Knob Hematite (PKH) deposit, which is located 240 m stratigraphically above the PKM and consists of variably mineralized bedded hematite and ore hosted in brecciated volcanic agglomerates. The PKM deposit was previously shown to be of magmatic and magmatic-hydrothermal origin, although its formation has not been precisely dated. The origin of the PKH deposit (i.e., sedimentary vs. hydrothermal) and its genetic relationship to the PKM, remain controversial.We present new U-Pb data on apatite intergrown with massive magnetite in the PKM deposit and provide the first precise age for the formation of the PKM ore at 1437.7 ± 5.8 Ma. Petrographic observations of PKH ore, bulk rock compositions, and the mineral chemistry of hematite, which contains up to 2.7% Ti, suggest that the hematite in the PKH deposit crystallized from acidic and hypersaline hydrothermal fluids at a temperature between 200 and 250 °C. The Fe isotopic composition of 9 bedded (δ56Fe = 0.05–0.30‰, average 0.13‰) and 3 brecciated hematite samples (δ56Fe = −0.19 to 0.01‰, average −0.06‰) from the PKH deposit are slightly lighter than the published δ56Fe results of magnetite from the PKM deposit (δ56Fe = 0.06–0.27‰, average 0.17‰). However, all isotopic signatures fall within the magmatic range, indicating that iron in both deposits was originally sourced from a magma. Because of the hydrothermal origin of the PKH deposit, the iron isotopic compositions of the PKM and PKH ores that imply a shared/similar iron source, and the spatial proximity of both deposits, we argue that the PKM and PKH deposits are genetically related and represent two endmembers of a high-to-low temperature magmatic-hydrothermal continuum. In this scenario, ore fluids exsolved from the magma that facilitated the formation of the PKM deposit migrated upwards, infiltrated existing sedimentary structures near the surface, and precipitated hydrothermal hematite ore while preserving the original bedded and brecciated structures.Geochemical signatures of the rhyolites/rhyodacites that host the PKM deposit imply that these rocks are A2-type felsic rocks that were emplaced in a post-collisional extensional setting. Bulk silicate Earth normalized patterns of the PKM deposit and wall rocks display a negative slope from Cs to Lu with negative Nb and Ta anomalies, indicating a hydrous source for the rhyolites and rhyodacites, possibly a subduction-modified subcontinental lithospheric mantle (SCLM). These geochemical signatures support a proposed tectonic setting of the St. Francois Mountains, wherein the igneous terrane developed on a growing continental margin. Episodic mafic-to-intermediate magmatism, and subsequently exsolved hydrothermal fluids, may have formed the cluster of IOA/IOCG-type deposits in the igneous terrane between ~1500 and ~1440 Ma. Within such a context, the PKM and PKH deposits may represent a shallow, small-scale snapshot of processes similar to the ones that form the IOA-IOCG continuum: a deeper magmatic event that exsolved a hydrothermal fluid that forms an overlying ore body.

On closed-form expressions for the approximate electromagnetic response of a sphere interacting with a thin sheet — Part 1: Theory in the frequency and time domain

In mineral exploration and geologic mapping of igneous and metamorphic terranes, the background is often dominantly resistive. The most important electromagnetic interaction is between a discrete conductor and an overlying sheet of conductive overburden (e.g., glacial clays or weathering products of the basement rocks). To enable the electromagnetic modeling of these common situations, here I provide closed-form expressions for the approximate electromagnetic response of a sphere embedded in highly resistive rocks and interacting with an overlying thin sheet. The sphere is assumed to be dipolar and excited by a locally uniform field. The expressions in the time and frequency domains are represented as sums of complete and incomplete cylindrical functions. New asymptotic approximations are provided for the efficient evaluation of the required incomplete cylindrical functions. The frequency-domain formulas are validated by numerical transformation to the time domain and comparison to the time-domain solution.

3-D kinematic restoration of the eastern Sudbury Igneous Complex, Canada, and its importance for Cu-Ni-PGE sulphide exploration

Kinematic restorations of deformed igneous terranes have rarely been attempted due to the general absence of (1) planar primary lithological contacts, (2) tightly constrained deformation kinematics, and (3) geometric constraints on lithologies and structures at depth. Because of the vast number of exploration drill holes, the deformed and layered 1.85 Ga Sudbury Igneous Complex (SIC) in Ontario, Canada, is reasonably well understood with respect to (1) and (3). We therefore explored the kinematics of folding and faulting in the eastern SIC and use the subsurface geometry of faults extracted from a model integrating surface mapping and drill core logs in an attempt to restore the geometry of part of the SIC. Specifically, we identified rotation magnitudes and the tilt axis of a kilometer-scale plunging fold, the West Bay Anticline, which appears to have folded the eastern part of the SIC. We combined a 3-D model of the faults and lithological contacts of the SIC, fault kinematics calculated from the inversion of brittle shear faults at surface, and limb rotations of the West Bay Anticline to kinematically restore the eastern part of the SIC. Results of restorations using simple shear and flexural-slip algorithms highlight the importance of slip on prominent, curved faults that displaced the SIC and facilitated its folding. Flexural slip appears to be the more plausible mechanism of the two, as it removed the curvature of, and displacement on, prominent faults. Flexural-slip restoration helps to estimate primary thickness variations of the SIC and the results show that thickened parts of the Main Mass of the SIC correspond spatially with known Cu-Ni-PGE sulfide deposits. This result demonstrates that 3-D restoration can be an important tool for mineral exploration in igneous terranes.

Alteration Mapping in Central Urumieh-Dokhtar Magmatic Assemblage of Iran Using ASTER and OLI Data

The study area is located on the Urumieh-Dokhtar Magmatic Assemblage of Iran, in the west of Ardestan between the East longitude 52°1' to 52°18' and the North latitude 33°17' to 33°27'. Remote sensing techniques are suitable for studying the alterations occurring in the igneous terranes. The alteration zones are well illustrated by implementation of the principal component analysis and the Crosta methods and Spectral Feature Fitting on ASTER data. In order to identify the lineaments, both Landsat-8 satellite imagery and GDEM-ASTER data are used in spatial processing. Using directional filtering and automatic extraction of lineaments, a tectonic lineaments map is prepared. Then alteration maps, tectonic lineaments map and 1:100,000 geology map are used to identify areas with high potential of Cu mineralization.

Mineralogical Investigation of the Bara Formation from Lakhra Anticline, Sindh province, Pakistan

The present study shows the bulk mineralogical composition of the Bara Formation from Lakhra anticline, near to Lakhra coalfield area of the Sindh province of Pakistan. The petrography, scanning electron microscope and X-ray diffractommeter were used for the detailed mineralogical investigations. Quartz, feldspars, hematite, gypsum, chlorite and goethite minerals were obtained from above methods. Quartz, feldspar and chlorite minerals indicated that the sediments were transported from igneous and metamorphic rocks and the hematite and goethite indicated the continental environment for the studied formation. Mineralogical investigation of the Bara Formation indicates that the sediments were supplied from the Igneous and metamorphic rock terrane. It can be said that the Bara formation sediments were supplied from the Igneous and metamorphic complex which is exposed in the southeastern part of the studied area.

Provenance of Late Paleozoic-Mesozoic Sandstones, Taimyr Peninsula, the Arctic

The sedimentary and provenance characteristics of seven Permo-Carboniferous and two early Cretaceous samples from the Taimyr Peninsula provide information about the latest evolution of Uralian orogeny and the opening of the Amerasian Basin. The Permo-Carboniferous samples have a mixed provenance of recycled and first cycle sediment, sourced from metamorphic and igneous terranes. U-Pb detrital zircon ages represent a mixture of Precambrian-Paleozoic grains with euhedral, penecontemporaneous late Carboniferous and Permian grains consistent with derivation from the Uralian Orogen, plus additional Timanian and Caledonian material presumably derived from Baltica. Differences between the late Permian sample and the other Carboniferous and early Permian samples are interpreted to reflect the final collisional stage of Uralian orogeny. Early Cretaceous sediments deposited at the time of the Amerasian Basin opening preserve a mixed provenance of mainly first cycle metamorphic and igneous source material, as well as an unstable heavy mineral assemblage dominated by staurolite, suggesting local derivation. Detrital zircon ages fall almost exclusively into one late Permian-early Triassic cluster, indicating a Siberia Trap-related magmatic source. The detrital zircon age spectra support a passive margin setting for Taimyr during the opening of the Amerasian Basin in the early Cretaceous.

The geochemistry and petrogenesis of the Paleoproterozoic Green Mountain arc: A composite(?), bimodal, oceanic, fringing arc

The inferred subduction affinity of the ∼1780-Ma Green Mountain arc, a dominantly bimodal igneous terrane (together with immature marine and volcaniclastic sedimentary rocks) accreted to the southern margin of the Wyoming province, is integral to arc-accretion models of the Paleoproterozoic growth of southern Laurentia. Conversely, the dominantly bimodal nature of many putative arc-related igneous suites throughout southern Laurentia, including the Green Mountain arc, has also been used to support models of growth by extension of pre-existing crust. We report new geochemical and isotopic data from ∼1780-Ma gabbroic and granodioritic to tonalitic rocks of the Big Creek Gneiss, interpreted as consanguineous with previously studied metavolcanic rocks of the Green Mountain Formation.The ∼1780-Ma Big Creek Gneiss mafic rocks show clear geochemical signatures of a subduction origin and provide no supporting evidence for extensional tectonism. The ∼1780-Ma Big Creek Gneiss felsic rocks are attributed to partial melting of mafic and/or mixed lower-crustal material. The bimodal nature of the suite results from the combination of arc basalts and felsic crustal melts. The lack of andesite is consistent with the observed tholeiitic differentiation trend of the mafic magmas. The lower ɛNd(1780Ma) values for the felsic rocks vs. the mafic rocks suggest that the unexposed lower crust of the arc may be older than the arc and that Trans-Hudson- or Penokean-aged rocks possibly form the substratum of the arc. Our results reinforce previous interpretations that arc-related magmatism played a key role in the Paleoproterozoic crustal growth of southern Laurentia, but also support the possibility of unexposed older crust as basement to the arcs.

Characterisation of igneous terranes by zircon dating: implications for UHP occurrences and suture identification in the Central Rhodope, northern Greece

The occurrence of ultra high pressure (UHP) and high pressure (HP) relicts associated with oceanic material suggests the presence of a suture zone within the Rhodope Massif. Characterisation of the accreted igneous terranes and their relationship with the UHP/eclogite occurrences provide new constraints on the location of this suture. Single-zircon evaporation and sensitive high-resolution ion microprobe dating of orthogneiss protoliths define two groups of intrusion ages: Permo-Carboniferous and Late Jurassic–Early Cretaceous. Structurally, the Late Jurassic gneissic complex overthrust a unit with Permo-Carboniferous orthogneisses. A “melange zone” marked by mylonites, eclogites, amphibolites, and UHP micaschists separates these two units. We interpret these observations in terms of two distinct igneous terranes, the Thracia (Permo-Carboniferous) and Rhodope (Late Jurassic) terranes, separated by the Nestos suture, and assembled during the closure of an oceanic basin of the Tethys. Geochemically, the Late-Jurassic rocks are akin to subduction magmatism, possibly the same subduction that caused the UHP metamorphism of metasediments within the “melange zone”. Observed UHP–HP relicts are restricted to the tectonic contact zone, suggesting that a single subduction/collison event can explain the occurrences of UHP relicts and eclogites in the Central Rhodope, and that subducted rocks are exhumed only within the Nestos suture.

Transition from arc- to post-collision extensional setting revealed by K-Ar dating and petrology: an example from the granitoids of the Eastern Pontide Igneous Terrane, Araklı-Trabzon, NE Turkey

The Eastern Pontide Igneous Terrane (EPIT) includes several Cretaceous to Neogene intrusive rocks, ranging in composition from low-K tholeiitic gabbros through calc-alkaline and high-K calc-alkaline metaluminous granitoids or peraluminous leucogranites to alkaline syenites. Such high diversity in age and composition is also accompanied by a broad spectrum in terms of geodynamics—i.e. from arc through syn-collisional thickening to post-collisional extensional regimes. Shallow-seated porphyritic acidic to intermediate rocks are from oldest to youngest, on the basis of field relations, the Gundogdu altered microgranite, the Bogali K-feldspar-megacrystic monzogranite and the Uzuntarla porphyritic granodiorite. These rocks, exposed in the southern part of the Arakli region, east of Trabzon, Turkey, were studied in terms of their mineralogy and petrography, whole-rock geochemistry and hornblende K–Ar dating. The mineralogical and geochemical data reveal an apparent diversity in incompatible-element enrichment and depletion, for the Bogali unit and Uzuntarla unit, respectively. The Bogali and Uzuntarla units yield hornblende K–Ar ages ranging from 75.7 ± 1.55 to 61.4 ± 1.47 Ma and from 42.4 ± 0.87 to 41.2 ± 0.89 Ma, respectively. The diversity in both mineralogy–geochemistry and hornblende K–Ar ages suggests that the Bogali and Uzuntarla units are parts of the Cretaceous arc and Eocene post-collision extensional-related igneous activity, respectively, in the EPIT of northern Turkey. Copyright © 2005 John Wiley & Sons, Ltd.

Stratigraphy, palynology, and provenance of the Colorado Creek basin, Alaska, USA: Oligocene transpressional tectonics along the central Denali fault system

New sedimentologic, biostratigraphic, and compositional data from a 415-m-thick section of siliciclastic and volcanic strata document Oligocene synthrusting sedimentation south of the McKinley segment of the Denali fault system. Strata of the Colorado Creek basin are presently exposed on the south side of the central Alaska Range in the footwalls of north-dipping thrust faults. New measured sections define a three-part stratigraphy. Lowermost strata consist of a ~30-m-thick unit of marine sandstone and mudstone that contain Late Cretaceous dinoflagellate taxa. The middle unit consists of ~330 m of conglomerate, sandstone, and mudstone interpreted as braided stream and floodplain deposits. This middle unit contains early Oligocene pollen and spore assemblages. The upper unit is 55 m thick and contains lava flows, tuff, and pumice interpreted as the product of subaerial volcanic eruptions. Direct age data are lacking from the upper unit. Compositional data from the middle unit indicate that detritus was derived from sedimentary and igneous source terranes exposed on both the north and south side of the McKinley fault. Matching source lithologies north of the McKinley fault with conglomerate clast types in the Colorado Creek basin implies 30–33 km of maximum post-early Oligocene dextral displacement along the fault. We interpret the Oligocene strata of the Colorado Creek basin as a product of transpressional deformation that produced north-dipping thrust faults associated with strike-slip displacement on the central Denali fault. Our data from the Colorado Creek basin, in combination with previous studies, document a major episode of middle Eocene – late Oligocene synorogenic sedimentation along the Denali fault from British Columbia to southwestern Alaska.

Geological factors affecting the distribution of trace metals in glacial sediments of central Newfoundland

In central Newfoundland (NTS 12A/10, 15, 16, 2H/1), As, Pb, and Zn concentrations in the clay-sized ( 1000 ppm), exceeding levels commonly set for purposes of environmental protection. Near Pb-Zn mines at Buchans, geochemical variation with depth reflects the dispersal of detritus from mineralized bedrock, and differences in sediment type and provenance. There, surface sediments are rich in granitic debris derived from the Topsails igneous terrane 5 km north of Buchans and contain low concentrations of trace metals. These sediments are compositionally unrelated to either Buchans Group volcanic rock or an underlying, older till enriched in sulphide minerals and trace metals. Metal-rich till extending up to 10 km southwest of Buchans results from combined glacial and debris flow transport related to two distinct geological events. Trace metals are enriched (two- to fourfold) in the clay-sized fraction of till compared to the silt and clay-sized, and are associated with Al- and Mg-bearing minerals that preferentially concentrate in the clay fraction. The geochemistry of the silt and clay-sized fraction can approximate that of the <2-mm fraction. Background variations in till illustrate the important role of a geological framework to the interpretation of geochemical surveys and the origins of trace metals in the environment.

Rayleigh Wave Group Velocity Tomography of Siberia, China and the Vicinity

—Rayleigh waves are used in a tomographic inversion to obtain group velocity maps of East Asia (40° E–160° E and 20° N–70° N). The period range studied is 30 to 70 seconds. Seismograms used for this study were recorded at CDSN stations, at a temporary broadband seismic array in Tibet, at several SRO stations, and Kirnos-equipped stations established in Asia by the former Soviet Union, in Siberia, in the Sakhalin and in Mongolia. Altogether more than 1200 paths were available in the tomographic inversion. The study area includes the Angara craton, the geologically ancient core of Asia, and the subsequently accreted units, the Altaids (a Paleozoic collision complex), the Sino-Korean platform (a chain of Archaen terranes separated by belts of active structures), the south China platform (a collage of Precambrian, Paleozoic and Mesozoic metamorphic and igneous terranes), as well as the Tibetan plateau (an active tectonic feature created in late Cenozoic through collision of the Indian subcontinent and the Asian continent). Many of these main units are recognizable in the tomographic images as distinctive units; Tibet appears as a prominent low velocity (about −15% from the average) structure, with western and central Tibet often appearing as the areas with the lowest velocities, the Central Asian fold-belt, and the Angara craton are consistently high group velocity areas. Some lesser tectonic features are also recognizable. For example, Lake Baikal is seen as a high velocity feature at periods greater than 40 seconds. However, the high group velocity feature does not stop near the southern end of Lake Baikal; it extends south-southwestward across Mongolia. The North China Plain, a part of the platform where extensional tectonics dominate, is an area of high velocities as a result of relatively thin crust. The south China block, the least tectonically active region of China, is generally an area of high velocity. For periods longer than 40 seconds, a NNE trending high group velocity gradient clearly exists in eastern China; the velocities are noticeably higher in the east. From the group velocity maps, average dispersion curves at twelve locations were determined and inverted to obtain velocity structures. Main results of group velocity inversion include: (1) a Tibetan crust of around 60 km thick, with low crustal and upper mantle shear velocities, at 3.3 km/s and 4.2 km/s, respectively; (2) with the Moho constrained at 40–43 km, the Angara craton and the Central Asian foldbelt have a VS in excess of 4.6 km/s; (3) relatively low shear velocities are obtained for tectonically active areas. In many parts of the study area, where Precambrian basement is exposed, the process in the crust and upper mantle due to recent tectonic activities have modified the crust and upper mantle velocity structures under the Precambrian terranes, they are no longer underlain by high velocity crust and mantle.

A Mid Cretaceous origin for the Galápagos hotspot: volcanological, petrological and geochemical evidence from Costa Rican oceanic crustal segments

The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66 Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hyaloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya (∼90 Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr–Nd–Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Galapagos hotspot. The volcanological, petrological and geochemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic results and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous (∼90Ma) age for the Galapagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Galapagos starting plume.

Quaternary geology of the Buchans area, Newfoundland: implications for mineral exploration

Mineral exploration for volcanogenic massive sulphide deposits in the Buchans area of central Newfoundland is hindered by thick and widespread Quaternary overburden comprising glacial, glaciolacustrine, glaciofluvial, and resedimented deposits. Mapping of striations and indicator erratics suggests that the area was affected by four ice-flow events: I, southward; II, northeastward and westward to southwestward; III, southward; and IV, northeastward and southwestward. Vertical and areal distributions of lead, copper, and zinc indicate that till and glaciolacustrine sediments overlying sulphide deposits can be enriched in mineralized debris. Near mines at Buchans surficial debris-flow deposits contain much material derived from the Topsails igneous terrane and are generally impoverished in mineralized debris. Southwest of Buchans, however, they can be enriched. Mineral exploration in the Buchans area must take into account that (i) sulphide-rich debris in till does not reflect only the youngest ice-flow event; (ii) glaciolacustrine sedimentation and resedimentation can result in burial of older sediments containing mineralized debris, in the redistribution of that debris, and in the geochemical masking of subcropping ore deposits; (iii) dispersal of sulphide debris during resedimentation is not necessarily related to ice-flow directions indicated by striations; (iv) a geochemical dispersal train originating at Buchans reflects both southwestward glacial transport and later redistribution of mineralized debris within debris-flow and glaciolacustrine deposits; and (v) marked geochemical variations with depth can occur with small variations in sampling depth, reflecting change in provenance and depositional process. The depositional and ice-flow model developed for the Buchans area likely has wider application in central Newfoundland.

Lithologic analysis from multispectral thermal infrared data of the alkalic rock complex at Iron Hill, Colorado

Airborne thermal‐infrared multispectral scanner (TIMS) data of the Iron Hill carbonatite‐alkalic igneous rock complex in south‐central Colorado are analyzed using a new spectral emissivity ratio algorithm and confirmed by field examination using existing 1:24 000‐scale geologic maps and petrographic studies. Color composite images show that the alkalic rocks could be clearly identified and that differences existed among alkalic rocks in several parts of the complex. An unsupervised classification algorithm defines four alkalic rock classes within the complex: biotitic pyroxenite, uncompahgrite, augitic pyroxenite, and fenite + nepheline syenite. Felsic rock classes defined in the surrounding country rock are an extensive class consisting of tuff, granite, and felsite, a less extensive class of granite and felsite, and quartzite. The general composition of the classes can be determined from comparisons of the TIMS spectra with laboratory spectra. Carbonatite rocks are not classified, and we attribute that to the fact that dolomite, the predominant carbonate mineral in the complex, has a spectral feature that falls between TIMS channels 5 and 6. Mineralogical variability in the fenitized granite contributed to the nonuniform pattern of the fenite‐nepheline syenite class. The biotitic pyroxenite, which resulted from alteration of the pyroxenite, is spatially associated and appears to be related to narrow carbonatite dikes and sills. Results from a linear unmixing algorithm suggest that the detected spatial extent of the two mixed felsic rock classes was sensitive to the amount of vegetation cover. These results illustrate that spectral thermal infrared data can be processed to yield compositional information that can be a cost‐effective tool to target mineral exploration, particularly in igneous terranes.

Sulfur isotopic characteristics of late Cenozoic ore deposits at the arc junction of Hokkaido, Japan

Abstract Sulfide minerals of late Cenozoic vein‐type deposits of southwest Hokkaido and Kuril Islands yielded δ34CDT S values of 2 to 8 permil, which are typical green‐tuff values of magnetite‐series igneous terrane. Sulfides of the Kitami district of northeast Hokkaido, on the other hand, are characterized by negative δ34SCDT values, ranging from 0 to ‐ 7 permil. This unique value among ore deposits in the late Cenozoic back‐arc terranes in the Japanese Islands is considered to have resulted from extraction of 32S enriched sulfur from the basement rocks, because of well‐developed N‐S fracturing in the basement, which is characteristic of the axial belt and Kitami district of Hokkaido.

Fan and playa areas in southern California and adjacent parts of Nevada

AbstractIn bolsons in the desert regions of southern California and adjacent parts of Nevada, the area underlain by alluvial fan gravels and playa sediments is generally ˜1·2 times the area being eroded to produce those sediments. In certain larger basins in the vicinity of Death Valley, however, the depositional area is only about half the size of the erosional area. This reflects the more active tectonic environment in these bolsons.Of the areas underlain by recent sediments in these bolsons, playas make up 2–6 per cent. Smaller playas are found in the Mojave region, and seem to be associated with sedimentary terranes. Conversely, igneous terranes support larger playas. Larger deviations of playa area from these averages are attributable to incorrect identification of bolson boundaries. Fine sediment is either able to pass through the bolson to the next down stream, or is being collected from areas upstream that were not considered to be part of the system. For example, the playa in Death Valley is unusually large. This is in part because the Death Valley playa has been deformed tectonically so parts of it are now eroding, and in part because the outlet of Lake Tecopa was downcut in the geologically recent past, so sediment once trapped there now reaches Death Valley. The size of the playa in Death Valley is still adjusting to these changes.

The Butler Hill Caldera: a mid-Proterozoic ignimbrite-granite complex

Abstract Uplift along rift-related faults in the eastern St. Francois igneous terrane of southeastern Missouri exposes a high-level anorogenic igneous complex consisting of 1.5 km of intracaldera ignimbrites, a zoned central pluton, and late-stage ring plutons. A collapse structure, named the Butler Hill Caldera (BHC), formed in response to eruption of comenditic ignimbrites of the Lake Killarney Formation which attains an intracaldera thickness of 400 m. Continued peralkaline volcanism resulted in an additional 1000 m of Grassy Mountain Ignimbrite within the caldera and a thin, but widespread, outflow sheet. Following degassing of the magma chamber, the central pluton forcefully intruded the intracaldera volcanic sequence producing arcuate folds concentric to the caldera axis. Remnants of the original magma chamber roof are not preserved. The final igneous event related to the BHC involved intrusion of amphibole-granitoids in an annular pattern within, and concentric to, the caldera walls. The latter bodies probably merge, with depth, into a single ring pluton. The petrographic zoning exhibited by the mildly peraluminous central pluton of the BHC consists of: (1) a roof-zone of fine-grained, brick-red, hypersolvus granophyre with abundant miarolitic cavities which is nearly devoid of hydrous minerals; (2) a transitional zone of pink fine- to medium-grained seriate granite in which granophyric intergrowths are common and plagioclase and biotite are present in trace amounts only; and (3) a border zone of medium- to coarse-grained pink to grey subsolvus biotite granite with rapakivi texture. The paradoxical association of peralkaline and peraluminous suites with hypersolvus and subsolvus petrographic traits at the same structural level is recognized in other high-level anorogenic complexes. The common factor in these terranes, regardless of their age, appears to be that initial magmas were hot, relatively dry, low in CaO and high in F. These compositional features favor ascent of hypersolvus magmas to subvolcanic levels where saturation is accomplished by influx of meteoric water. Field, chemical, isotopic, and petrographic evidence indicate that BHC rocks have experienced at least two periods of subsolidus metasomatism. The resulting metasomatic features are typical of anorogenic ignimbrites and high-level granitoids elsewhere and must be quantified before existing chemical data can be applied to questions of igneous petrogenesis.

Influence of a rigid block on rift-margin evolution

The Middle Proterozoic St. Francois igneous terrane of southeast Missouri is located on the northwest margin of the Reelfoot rift and was modified into a northeast-trending structural block during periods of Late Proterozoic-Early Cambrian extension and shoulder uplift. Late Cambrian facies patterns indicate that the St. Francois terrane was not a passive basement feature during early periods of thermal subsidence of the Reelfoot rift. Facies relations also show that basement response to syndepositional tectonics is the principal control on rift-margin geometry, and not sedimentary loading as assumed by models. Syndepositional tectonics that influenced southeast Missouri during Late Cambrian time are interpreted to be a result of in-plane stress related to changing regional stress fields.