121 research outputs found
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The detrital zircon geochronology and structural petrology of Pacheco Pass, Diablo Range, central California : implications for subduction zone timing and tectonics
No full textThe timing and tectonic history of blueschist facies rocks, though a widely studied problem, is still a topic of considerable debate because of poor age constraints in terranes with few fossils or marker beds. This study applies U-Pb detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), to a well-studied blueschist facies terrane, the area around Pacheco Pass, which is a window through the Great Valley forearc into the deepest coherent part of the Franciscan accretionary complex. 150 zircon grains were analyzed from each of sixteen samples, to determine their U-Pb ages. Additionally, a mineralogical and microstructural study of 103 thin sections was performed by traditional optical microscopy to supplement the geochronologic information. The maximum ages of deposition for coherent metagraywackes are as young as ca. 90 Ma, and ca. 70 Ma for blocks in mélange. Using the observed age distribution, the coherent rocks are divided into two units: Unit 2 (unimodal) and Unit 3 (bimodal). Unit 2 is structurally higher than Unit 3; thus, it was underplated first. Similar patterns are present in the Sacramento Valley of the Great Valley forearc basin (DeGraaff-Surpless et al., 2002), except that the base of the section is unimodal, and the upper parts are bimodal. This evolution is thought to reflect increased dissection of the arc over time due to the eastward stepping of the drainage divide. The source area for the coherent units is inferred to be similar to that of the Great Valley Group: the Sierran magmatic arc and associated terranes in northern California, western Nevada and southern Oregon. To explain the geochronology, as well as field observations, underplating must occur at the tens-of-meter scale, although larger or smaller packages of rocks are not precluded from being underplated. Minimum rates of underplating, based on the maximum ages of deposition for the two coherent units, are 10 to 30 m/m.y. The layer-parallel foliation is defined by sheet silicates and pressure solution selvages, with sodic amphibole, lawsonite and blocky jadeitic pyroxene parallel to it. The foliation formed immediately after underplating, and the rocks were affected by high shear strain in the subduction zone, evidence of which is seen in the phyllite (for example, cm-scale isoclinal folds). Near-static conditions were reached by the time continued underplating sequestered the rocks from the subduction channel. The crystallization of radiating sprays of jadeitic pyroxene and unoriented lawsonite tablets occurred at this time. These conditions are also recorded by the opening of anisotropy-controlled layer-parallel veins. Mélange genesis began after 90 Ma, and was active by at least 70 Ma, which corresponds to the onset of Laramide style orogenesis in western North America. By 60 Ma, however, the rocks were at 110°C based on apatite fission track data, which corresponds to a depth of ~15 km, with geothermal gradients of 10°C/km. This indicates rates of unroofing of 500 m/m.y. from depths of ~30 km. Localized folding and simple shear in an extensional regime is characteristic of this latter phase of deformation. Folds with axial planar pressure solution cleavages that are superposed upon one another indicate localized zones of sinistral shear in near-vertical shear zones. Ptygmatically folded veins have been shortened up to 70% in some cases. Pressure solution has removed up to 30% of the rock volume in some cases. When the rocks moved out of the regime of ductile deformation, layer-perpendicular veins and faults formed. Some of the unroofing, therefore, may have been accommodated by faulting in the latter part of the unroofing history. Based on the Quien Sabe volcanic rocks (11-9 Ma) that overlay Franciscan rocks of the Diablo Range, the rocks must have been at the surface by ~10 Ma. Therefore, the minimum rate of unroofing between 60-10 Ma is 300m/m.y. This indicates slow synsubduction unroofing.Earth and Planetary Science
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Veins and alteration envelopes in the Grasberg Igneous Complex, Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia
No full textThe Grasberg Igneous Complex (GIC) consists of three main phases of igneous activity: the Dalam Igneous Complex, the Main Grasberg Intrusion, and the Kali Intrusion. Each contains veins revealing a history of fluid flow that has concentrated minerals of economic value. A generalized sequence of early magnetite ± quartz veins followed by quartz ± sulfide/oxide veins followed by late chalcopyrite/pyrite veins is observed in the Dalam Igneous Complex and Main Grasberg Intrusion. The youngest igneous body, the Late Kali Intrusion, cross-cuts the older igneous bodies as well as their veins, and has biotite ± quartz, quartz ± pyrite and pyrite ± quartz veins. Pyrite ± quartz veins with alteration envelopes up to 14 cm total width are found in regions of the complex that are higher in elevation and distant from the center of copper mineralization. Geochemical analyses of wall rock and alteration envelopes from eleven samples are compared to determine which components were added to the altered samples and which were removed by the fluids. Most major components were removed by fluids (Na₂O, MgO, SiO₂, CaO, FeO, and TiO₂) along with many trace elements (Cu, Cl, Ga, Rb, Sr, Nb, Ba, Y, Zr, Eu, Yb, Tl, Au, La, Th, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Tm, and Lu). Gain or loss of K₂O and P₂O₅ vary depending on the sample. H₂O and S were added to the altered wall rock. Typical host-rock mineral assemblages include plagioclase, biotite, quartz, alkali feldspar, pyrite, chalcopyrite, and magnetite. Typical alteration envelope mineral assemblages include muscovite, alkali feldspar, pyrite, and quartz. Balanced reactions between wall rock minerals and fluid to produce alteration minerals typically involve the consumption of HCI, indicating that the altering fluids had a low pH. The alteration envelopes are believed to be the result of changes as fluids flowed through the complex, including decreasing temperature, generation of HCI by the precipitation of pyrite and chalcopyrite from copper and iron chlorides in the lower and central parts of the complex, and/or the decrease in the fluid prewall rock fluid pressure surrounding veins. Scanned cathodoluminescence of quartz in quartz-sulfide veins reveals detailed textures on the scale of tens to hundreds of microns including concentric growth zoning and fractures. Growth textures indicate that the quartz grew into open space, so these veins remained open during infilling. Vein growth is believed to have occurred from fluids that flowed through the veins. Microfracturing occurred after the veins began to close. Experimental studies of Cline and Bodnar (1991) applied to fluid exsolution from magma chambers are used as a basis to explain the sequence of veining. Fluid separating from a magma at low pressure (<1 kilobar) has initially low concentrations of copper, whereas fluid separating from a magma at high pressure (≥2 kilobars) has initially high concentrations of copper. Crystallization from a deep batholithic magma chamber at depths greater than 6 km with a molten stock reaching up to shallower depths (less than 3 km) can account for the changes in copper precipitation observed in the GIC system over time. Early crystallization in a stock at shallow depths led to exsolution of an early fluid that was relatively copper-poor. This resulted in early magnetite ± quartz veins. Deeper-seated crystallization eventually generated copper-rich fluids forming chalcopyrite/pyrite veins. Finally, the latest stages of veining following the Late Kali Intrusion were relatively copper-poor due to the last fluids exsolving from the deeper copper-depleted part of the magma chamberEarth and Planetary Science
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Geochemical and thermal insights into caldera-forming "super-eruptions"
No full texttextExplosive, caldera forming "super-eruptions" (an eruption of VEI 8 or larger, resulting in 1000+ km³ of volcanic ejecta in ignimbrite sheets) are the single most destructive natural disaster native to Earth. Super-eruptions require three elements to occur: 1-crustal magmatic fluxes above background solidification rates, 2-growth of a batholith scale magma chamber, and 3-an eruption trigger. This study addresses these requirements with new petrographic and geochemical analyses and numerical simulations of crustal magma bodies. Crustal magmatic fluxes up to 10x steady-state arc rates are required to form volcanic provinces that host super-eruptions. Super-eruptions can occur in continental hot-spots or rift environments. Why arcs "flare-up" is the subject of active debate. Arcs may follow a regular cycle of lithospheric thickening, delamination, and asthenospheric upwelling (the Andean cycle); alternatively fertilized lithospheric mantle may undergo rapid melting. Targeted sampling (n = 165) of mapped but unsampled mafic and lamprophyric magmas in the San Juan magmatic locus of Colorado, an archetypical ignimbrite province, over three years identified both the lithospheric mantle reservoir and the most primitive San Juan magmas using optical petrography, whole rock geochemistry (n = 50) and Pb, Sr, and Nd isotope geochemistry (n = 32). These mafic magmas more closely resemble the continental lithosphere geochemically. Mixing models based on Energy Constrained Assimilation/Fractional-Crystallization (EC-AFC) indicate that the San Juan magmatism is the product of lithospheric melts and 30-40% crustal assimilation rather than asthenospheric upwelling. The Farallon flat-slab "pre-fluxed" and refrigerated the Colorado lithospheric mantle; removal of that slab at around 40 Ma triggered the SJVF "flare-up." Numerical simulations of crustal magma chamber growth indicate giant magma chambers form when high magma fluxes raise upper crustal temperatures to 300-400 °C at 5-10 km depth. These simulations focus on chamber growth, convection, and cooling at the expense of geometry or chamber mechanical failure with realistic sill-like geometry at the expense of thermal modeling. New 3D finite difference simulations emphasize the importance of geometry on chamber lifespan and crustal heating. A spherical chamber (i.e. model construct) requires 10x the cooling time of a 2km caldera footprint sill of same volume. Increasing sill thickness by 1km can double chamber longevity. Focused intrusions (i.e. 1D modeling) locally produce higher thermal gradients and preserve larger primary basalt volumes. Random intrusions in 3D yield basalt to crust ratios of 3-4:1 (required in the EC-AFC models). Random intrusion in 3D into the upper crust at "flare-up" fluxes ([greater than or equal to]10 km³ per k.y.) elevate average crustal geotherms by 10 °C / km, allowing for growth of batholithic scale magma chambers a wider footprint. Once situated in the upper crust, sub-caldera magma chambers cool inward forming moving crystallization and fluid saturation fronts. If the saturation front propagates faster than the crystallization front, nucleating fluid bubbles have the opportunity to grow, ascend, and collect at the chamber roof. New 2D finite difference models couple magma chamber cooling to fluid production to explore the conditions of fluid escape and collection. Less silicic magma composition, equant geometry, high ambient thermal gradient, and a stock all aid in fluid pocket growth by slowing the advance of the crystallization front (a fluid trap) and triggering saturation at lower fluid concentrations. Fluid pockets that grow to certain sizes ( > 500 m hemispherical bubble) have the potential to trigger an eruption by propagation of a fluid fracture to the surface. This mechanism possibly triggered the eruption of the 5000+ km³ Fish Canyon Tuff as well as smaller, recent eruptions (Pinatubo, El Chichón). Caldera forming super-eruptions occur in regions that meet these three requirements: 1-high magmatic flux, 2-rapid growth to batholithic size, and 3-a delayed eruption trigger. For the SJVF of Colorado melting of the "pre-fluxed" lithosphere provided the magmatic pulse which melted and heated the crust, forming a broad batholith. As magmatism peaked and began to wane, upper crustal magma chambers started to crystallize, exsolving fluids. These fluids ascended, collected, and fractured their way to the surface, triggering the Fish Canyon Tuff and other eruptions.Earth and Planetary Science
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Distribution of heat production in two metamorphic core complexes, Basin and Range province, Arizona : quantitative constraints on models of regional thermal structure
No full textThe amount and distribution of crustal heat production is a vital component of all estimates of continental thermal structure, yet it remains in most cases an assumption rather than a constraint. This study utilizes two Cordilleran metamorphic core complexes, the Catalina core complex and the Harquahala Mountains, as large and extensive exposures of the recent (<3O Ma) upper and middle crust of the southern Basin and Range of Arizona to gather primary heat production data. The depth distributions obtained do not follow a smooth or systematic function as predicted by models developed from the interpretation of linear relationships between surface heat flow and heat production; instead, they reflect a primary control exerted by the local structural or magmatic history. In each core complex, the amount of heat production observed plus that inferred to reside in the deeper crust is approximately 50% higher than predicted by standard estimates. This result is corroborated by variogram analysis of the data combined with published stochastic models. The difference results in overestimation of deep crustal temperatures by 150°C or more. Two-dimensional conductive thermal models of an evolving metamorphic core complex are utilized to provide insights into syn- and post-extensional thermal history. Model predictions of ancient cooling paths are remarkably consistent with estimates based on both structural and thermochronologic constraints, to the extent of reconciling seemingly conflicting data sets. The present-day heat flow patterns and low-relief Moho are most closely matched by a balanced geometry in which unroofing of the core complex is compensated by pure shear extension off-center, which may be analogous to models in which lower crustal flow compensates for tectonic denudation. The net amount of extension observed in Arizona core complexes does not appear to be sufficient to explain the high heat flow which characterizes the overall southern Basin and Range, indicating that additional sources of heat may be required. A new technique for determining concentrations of heat-producing elements in natural samples by gamma-ray scintillation spectrometry includes a test and partial correction for secular disequilibrium effects.Earth and Planetary Science
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Petrology of the southwest margin of the Grasberg Igneous Complex, Papua, Indonesia
No full textThe 3 Ma Grasberg Igneous Complex (GIC) of Papua, Indonesia is host to a super-giant porphyry copper-gold deposit. The GIC is shallowly emplaced into folded and faulted limestones that are as young as late Miocene. The Heavy Sulfide Zone (HSZ) is a pyrite-rich shell that surrounds the GIC. The HSZ grades into the Marginal Breccia. Near the surface the Marginal Breccia is overlain by the Banded Clay, a halloysite-clay rich unit. This study has determined the relationship of these units and characterized the pervasive alteration from the petrology of 588 samples collected from five drill cores and 23 outcrop samples. The initial Dalam phase of intrusion generated ~5 m of skarn (epidote, garnet, and clinopyroxene) at the margin of the GIC below 3700 m elevation. The precipitation of magnetite in the core of the GIC, the hydrolysis of SO₂, and the precipitation of pyrite in the periphery of the deposit generated abundant HCl and H₂SO₄. In the outer GIC, primary igneous minerals were altered to sericite, quartz, and pyrite by the acidic hydrothermal fluids. These fluids dissolved carbonate host rock and precipitated abundant pyrite. Layered dissolution infill indicates porosity was locally very high. High permeability along the margin of the GIC would have channeled hydrothermal fluid upwards. As supporting carbonate rock was dissolved forming the Marginal Breccia, open fractures formed along the contact with the GIC. Pyrite precipitated into these fractures, forming layering in places. Unsupported pieces of igneous rock and skarn broke off and fell into openings which became filled with pyrite. This process caused most of the growth of the HSZ to progress inward, leaving rocks that were once at the margin of the GIC separated from it by lenses of massive pyrite. Minor sphalerite and galena precipitated at lower temperatures at the outer edge of the HSZ. Only trace amounts of copper sulfides are present within the pyrite-rich HSZ because nearly all the copper was precipitated in the center of the deposit. Any H₂S or SO₂ remaining in the upwelling fluids mixed with oxygenated meteoric waters forming concentrated H₂SO₄, which intensely altered volcanic sediments forming the Banded Clay. The dissolution-enhanced permeability along the contact zone facilitated the drainage of magmatic fluids. This acted to increase the fluid pressure gradient driving pervasive infiltration between the center and the margin of the GIC. This was a significant contributing factor to the creation of the super-giant orebody. The dissolution of carbonate wall-rock caused the GIC to collapse outwards. This caused extension fracturing within the GIC that diverted fluid flow upwards and back into igneous rock. As a result some of the outermost igneous rock in the upper part of the GIC is relatively fresh, with primary groundmass orthoclase and plagioclase phenocrysts. The outward collapse of the GIC generated the flaring-upward shape that is a distinct attribute of the Grasberg system.Earth and Planetary Science
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Stratigraphy, structural geology, and tectonics of a young forearc-continent collision, western Central Range, Irian Jaya (western New Guinea), Indonesia
No full texttextNew Guinea has long been recognized by geologists as the location of geologically recent mountain building. This study combined field mapping, stratigraphic and remote sensing analysis along and near the Gunung Bijih (Ertsberg) mine road and mining district in order to analyze the geologic development of the collisional New Guinea orogen. As a result of the youthfulness and the quality of data, it is possible to constrain distinct parts of orogenic evolution to 1 or 2 m.y. The southern Central Range of New Guinea is located on the northern Australian continental margin. The southern one-third of the Central Range, exposed along the Gunung Bijih mine access road, is a 30-km-wide, north-dipping homocline exposing an apparently 18-km-thick Precambrian or Early Paleozoic to Cenozoic sequence. Following rifting in the early Mesozoic and until the Middle Miocene, the northern Australian continent was a passive margin. The Central Range of Irian Jaya formed when the Australian passive margin was subducted beneath and collided with a north-dipping subduction zone in the Middle Miocene. Litho- and biostratigraphic analysis of the New Guinea Limestone Group in the Gunung Bijih mining district and regional stratigraphic correlation indicates that the first evidence of subaerial exposure and erosion of the orogen is the widespread deposition of siliciclastic, synorogenic strata at ~12 Ma. I name this event the Central Range Orogeny. There is no evidence of an Oligocene orogenic event in the Irian Jaya region as has been described to the east in Papuan New Guinea. Deformation in the Central Range is dominated by ~12 to ~4 Ma southwest verging (210°-220°) contraction and minor east-west wrenching. This deformation is equally accommodated, there is no evidence for strain partitioning in the Central Range. Lithospheric-scale cross sections, incorporating field observations, predict the Central Range Orogeny is divided into a pre-collision and collisional stage. The pre-collision stage is the bulldozing of passive margin sediments in a north dipping subduction zone. The collision stage occurs when buoyant Australian lithosphere can not be subducted. The collision stage results in basement involved deformation and lithospheric delamination of the already subducted Australian plate.Earth and Planetary Science
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Kinematic analysis of outcrop-scale structures, southern Big Sur segment of Highway 1, Monterey and San Luis Obispo Counties, California
No full textThe Nacimiento Block is located in the Southern Coast Ranges of California, and consists mainly of Franciscan Complex accretionary prism rocks. It is cross-cut by the San Gregorio-Hosgri Fault Zone, a major right-lateral strand of the San Andreas Fault System. The Nacimiento Block is bounded on the east by the Nacimiento Fault, of debated timing and kinematics, which separates it from the Salinian Block. The Salinian Block is a piece of the Sierra Nevada Batholith, and both the Salinian and Nacimiento Blocks have been displaced from southern California by right-lateral slip on the San Andreas Fault System. To address the question of fault kinematics, a 48 kilometer long section of the Nacimiento Block was examined along California Highway 1 between Lopez Point and Ragged Point. Exposure occurs along approximately 20 kilometers of the transect, and landsliding obscures approximately half of the exposure. The remaining 10 kilometers of outcrop were mapped. Kinematic data were taken on 29 outcrops, totaling 542 minor faults, 406 with slickenlines and 258 with sense of slip indicators, along with 314 veins. Of the faults, 202 are dip-slip (60-90° rake), 113 are oblique-slip (31-59° rake), and only 91 are strike-slip (0-30° rake). The dominant mode of minor faulting is normal, with 111 faults observed, compared to 25 reverse, 24 left-lateral, and 28 right-lateral strike-slip. Two sets of vein and one set of dike orientations were measured. Stereographic analysis reveals the normal and reverse faults dip steeply to the southwest and strike northwest-southeast, subparallel to the coast and San Gregorio-Hosgri and Nacimiento Faults. There is no dominant orientation to the strike-slip faults. Faults of all types cut 17 slab-window related andesitic dikes, which are likely Early Miocene in age according to apatite and zircon fission track ages. The character of all fault planes is similar, indicating they are coeval. Three stages of deformation are recognized. Subduction generated mélange, the dominant lithology in this area, and "broken formations". A second stage of deformation is recorded in the emplacement of dikes and one set of veins. A third stage of deformation is recorded in the minor planar faults that were measured in this study. It is proposed that this latest phase of deformation is caused by the gravitational collapse of the western edge of the Santa Lucia Range. The normal faults parallel the coastline and local slope angles are up to 40°. Coeval strike-slip associated with the San Gregorio-Hosgri Fault Zone is superimposed on this deformation. Apatite fission track ages (n=3) indicate that the dikes mapped along Highway 1 cooled to 110°C at approximately 11 Ma. This indicates an unroofing rate on the order of 300 m/my. This anomalously fast unroofing is accomplished by side-inwards gravitational collapse and erosionEarth and Planetary Science
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Structural analysis of the San Simeon fault zone, California : implications for transform tectonics
No full textThe San Gregorio-Hosgri fault zone (SGH), located in the Southern Coast Ranges of California is a 420 kilometer long right-lateral strand of the San Andreas fault system. The San Simeon fault zone is a segment of the SGH that cross-cuts the Nacimiento block which is primarily composed of Franciscan Complex accretionary prism. The Nacimiento block is juxtaposed against the Salinian block, a portion of the Sierra Nevada batholith, by the Nacimiento Fault. The Nacimiento and Salinian blocks have been displaced from the south in a right lateral sense as part of movements within the San Andreas fault system. The San Simeon segment juxtaposes mid-Jurassic Coast Range Ophiolite with Cretaceous Franciscan accretionary prism material. These units are locally overlain by the Oligocene Lospe Formation and Miocene Monterey Formation. To better understand the movement history near the San Simeon fault zone, 33 kilometers of outcrop were examined along the sea-cliff between Ragged Point in the north and Pico Creek to the south. Of this transect, 4 kilometers were buried under marine terrace and sand dunes. No data was collected along 1 kilometer of transect due to the presence of elephant seals. The 28 kilometers of bedrock examined include: 7 kilometers of ophiolitic material, 16 kilometers of Franciscan Complex, 2 kilometers of Lospe Formation, and 3 kilometers of Monterey Formation. In all, 466 minor faults and 254 major (≥0.5 meters exposure length) faults were mapped, and 22 of these major faults juxtapose different formations (n=8) or different units within the ophiolite (n=14). Slickenlines were measured on 517 faults, of which 237 record sense of slip. Of the faults measured, 199 are strike-slip (0-30° rake), 179 are dip-slip (60-90° rake), and 139 are oblique-slip (31-59° rake). Sense of slip indicators record a wide range of movements: 49 right-lateral, 47 left-lateral, 40 normal, 38 reverse, 18 reverse left-lateral, 17 normal left-lateral, 15 normal right-lateral and 13 reverse right-lateral faults. The study transect was divided into structural domains based on fault kinematic patterns. Movement recorded in these data resulted from transform-related faulting. Fault kinematics that differ from the regional N35W strike of the San Simeon fault zone are explained by local variations in movement patterns near the San Simeon fault zone. This variations include local bends and splays off of the fault zone. The Lospe and Monterey Formations that make up 18% of the mapped transect contain 12% of the faults. These formations only experienced transform-related deformation. Faults in the Monterey Formation are parallel to the regional San Simeon fault zone. Faults in the Lospe Formation to the north primarily strike E-W. Ophiolite material contains 25% of the mapped transect and 37% of the faults. These faults primarily indicate right-lateral movement; however, reverse and normal faulting near perpendicular to the regional NW fault trend is common. The Franciscan Complex along 57% of the mapped transect contains 51% of the faults. Faults in the Franciscan Complex and the ophiolite potentially record subduction-related faulting, but evidence from fault kinematics from this study indicates transform-related faulting. Reverse and right-lateral faulting along the splays is indicated. East of San Simeon Point, a 1 kilometer wide San Simeon fault zone is indicated by a cluster of faults between the San Simeon Pier and Broken Bridge Creek, the eastern boundary of the fault zone. The complexity of fault patterns and kinematics in and near the San Simeon fault zone record a long and complex history of transform faulting.Earth and Planetary Science
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Depositional periodicity and the hierarchy of stratigraphic forcing in the Triassic carbonates of the Dolomite Alps, N. Italy
No full texttextThe Dolomite Alps of northern Italy are a classic field locality in the development carbonate stratigraphic theory. Included in the many discoveries rooted in the geology of the Dolomites is the concept of a hierarchy of stratigraphic forcing in the Alpine Triassic. The hierarchy states that carbonate sedimentation is dominantly a record of eustasy, resulting in organized stacking patterns, and that these stacking patterns reflect the interplay between low frequency (1-10 my) eustatic cycles and their component bundled high-frequency (100 & 20 kyr) eustatic cycles. The overall aim of this study is to further investigate the validity of the hierarchical model after recent dating of Anisian and Ladinian successions called the Milankovitchian periodicity and/or allocyclicity of the cyclic series into question. The study was completed using four sub-studies, 3 based on data collected in the field and a fourth based in cycle theory and computer modeling. First, it can be shown that allocyclic forcing exists in the Anisian/Ladinian platforms of the Dolomites by comparing the stratigraphic sections measured from 2 time-equivalent, independent carbonate platforms, the Latemar and Mendola Pass. Second, computer modeling of Anisian/Ladinian carbonate platform stratigraphy using Milankovitchain solar insolation as a proxy for high-frequency eustasy shows that both pure Milankovitch forcing and mixed Milankovitch/sub-Milankovitch forcing will produce synthetic carbonate platforms with stratigraphic successions comparable to those of the Anisian/Ladinian platforms of the Dolomites. Third, it can be shown that the while the Norian Dolomia Principale (a regional carbonate shelf) was affected by differential subsidence, megacycles systematically increase in their number of component cycles from 2-3:1 in the eastern Dolomites (updip) to 5-6:1 in the western Dolomites (seaward). In conclusion, the concept that carbonate platform stratigraphy is a record of an interplay between eustasy, subsidence, and sedimentation is upheld, while the validity of Milankovitchian forcing acting on all Alpine carbonate cycles is questioned. Instead, cyclic carbonates with sub-Milankovitch periodicities were common in the early and mid-Triassic, while cycles with Milankovitchian periodicities were common in the late Triassic.Earth and Planetary Science
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Hidden intrusions and molybdenite mineralization beneath the Kucing Liar Skarn, Ertsberg-Grasberg Mining District, Papua, Indonesia
No full texttextThe Ertsberg-Grasberg Mining District of Papua, Indonesia (Western New Guinea) hosts the Ertsberg Cu-Au Skarn, the giant Grasberg Porphyry Cu-Au deposit, and several other orebodies. Two 1700-meter-long cores beneath the Kucing Liar ore skarn (KL98-10-22) and the Grasberg Igneous Complex (KL98-10-21) contain high concentrations of vein and disseminated molybdenite. KL98-10-22, the focus of this study, intersects two previously unencountered intrusions, the “Tertiary intrusion Kucing Liar” (Tikl) and “Tertiary Pliocene intrusion” (Tpi). An intense dilatational quartz vein stockwork cuts Tikl and Ekmai Sandstone (Kkes) units, predating Tpi intrusion. Prior to these ultradeep cores, which extend almost 3 km below pre-mining surface, molybdenite was rarely observed in the district.
Geochemistry and isotopic data indicate that Tikl and Tpi intrusions originated from the same large magmatic system that emplaced other ore-forming Ertsberg-Grasberg district intrusions. Magma in a lower crustal chamber was recharged at least twice, according to Sr-Nd data. Laser-ablation inductively-coupled plasma mass spectrometry of magmatic zircons yields 238U-206Pb ages between 3.40 ± 0.12 Ma (Dalam Andesite) and 2.77 ± 0.15 Ma (Ertsberg intrusion), revealing a shorter period of igneous activity than previously measured by K-Ar and Ar-Ar dating. Analyses include composite ages of 3.28 ± 0.08 Ma for Tikl and 3.18 ± 0.11 Ma for Tpi. Inherited zircon cores indicate Precambrian (mostly Proterozoic) basement.
Molybdenite veining beneath the Kucing Liar Skarn and Grasberg Igneous Complex postdates stockwork veining and occurred before the 2.99 ± 0.11 Ma Kali dikes. Only one molybdenite vein was observed cutting Tpi. Molybdenites yielded ~3 Ma Re-Os ages and anomalous >4 Ma and <0.5 Ma ages; anomalous ages were not reproducible in follow-up analyses (this study). Smearing deformation of molybdenite (through fault activity) causes crystal strain, likely leading to annealing recrystallization. Recrystallization possibly redistributes daughter-product Os, resulting in anomalous ages from annealed material. Fluids with high Mo/Cu ratios (which were likely supercritical) precipitated late-stage molybdenite deep in the system. These fluids developed through magma chamber crystallization, which concentrated molybdenum in the melt as an incompatible element, and stripping of Cu from the magma chamber during hydrothermal activity.Earth and Planetary Science
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