Northeast-trending Belt Research Articles

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest−southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast−southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Deformation history and tectonic significance of the Sanagak Lake shear zone, Boothia Peninsula, Nunavut

The recently recognized Sanagak Lake shear zone (SLsz) is a 165 km long, southwest striking corridor of high-strain rocks that transects the southern portion of Boothia Peninsula, Nunavut. This zone records pervasive deformation (DSL1) at conditions of ∼0.52 GPa and ∼700 °C, and localized deformation (DSL2) at ≥0.5 GPa and 300–500 °C that preserve left lateral and right lateral senses of movement, respectively. Neocrystallized DSL1 titanite in a hornblende-bearing granodiorite yields an age of 1804 ± 6 Ma, interpreted to be the timing of DSL1. The timing of DSL2 is loosely bracketed by 40Ar/39Ar hornblende (1814 ± 3 Ma) and biotite (1743 ± 1 Ma) cooling ages since the deformation temperature falls between the estimated closure temperature of these minerals. Similar rock types and metamorphic conditions on either side of the shear zone rule out the SLsz as a terrane boundary. Rather, strain localization may have been triggered by thermal softening related to the emplacement of a northeast-trending belt of high-temperature granites south of the shear zone between 1840 and 1820 Ma. Deformation and metamorphism at ca. 1.81 Ga south of Boothia Peninsula and in the central Rae (Committee Bay belt) have been attributed to the Superior Province colliding with the southeastern margin of the Rae craton, such that the SLsz may too have formed in response to far-field stresses derived from this collision. The absence of ca. 1.81 Ga tectonic fabrics north of the shear zone indicates that the SLsz marks the northwestern extent of mid-crustal, Trans-Hudson related tectonometamorphism.

Within-Plate Setting of Paleozoic Alkalic Suites in Southeastern New England, USA: Constraints from Chemical Abrasion–TIMS U-Pb Geochronology and Paleomagnetism

AbstractMid-Paleozoic alkalic plutons and associated volcanic rocks in a northeast-trending belt transecting the southeastern New England segment of “West” Avalonia are well known for their anoroge...

Early Jurassic A-Type Granites in Southeast China: Shallow Dehydration Melting of Early Paleozoic Granitoids by Basaltic Magma Intraplating

AbstractA-type granites are widespread in most orogenic belts, and their petrogenesis has played an important role in reconstructing orogenic events. However, their sources, even today, are a subject of considerable controversy. At least four models have been proposed for their formation, namely, melting of a previously melted source, fractionation of basaltic magma, anatexis of crustal igneous sources, and magma mixing. Here we present Early Jurassic A-type granites in southeast (SE) China as an example of a shallow (ca. 15–20-km depth), igneous, crust-derived A-type granite. We present detailed laser ablation ICP-MS zircon U-Pb dating and major- and trace-element and Sr-Nd isotopic data for these granites and assess their origin and relationship to the geodynamics of the late Mesozoic extensive magmatism in SE China. The associated granites include the Zhaibei, Keshubei, Zhulanbu, and Guangze plutons, which were emplaced during the interval 191–188 Ma and occur in a northeast-trending belt, parallel to ...

Multi-chronometric dating of the Huarong granitoids from the middle Yangtze Craton: Implications for the tectonic evolution of eastern China

The tectonic–thermal history of the Huarong granitoids is used to clarify the tectonic evolution of the middle Yangtze Craton and eastern China. The Huarong granitoids include the Xiaomoshan and Taohuashan plutons and form a northeast-trending belt in the middle Yangtze Craton, eastern China. Multi-method geochronology reveals the thermal history of the Huarong granitoids from their emplacement to near-surface conditions. LA-ICP-MS U–Pb zircon dating suggests granitoid emplacement at ca. 117Ma; a cluster of zircons at ca. 127Ma is likely inherited from an earlier intrusion phase. The granitoids formed in an extensional setting, which was almost certainly associated with Pacific Plate subduction in the late Early Cretaceous. Biotite 40Ar/39Ar ages at ca. 96Ma record regional post-magmatic cooling. Apatite fission-track (AFT) and apatite (U–Th–Sm)/He (AHe) thermochronology and thermal-history modeling indicate cooling in three stages: enhanced cooling at 96–60Ma is interpreted to reflect cooling from the late Early Cretaceous regional magmatism. Tertiary (ca. 60–20Ma) very slow cooling, stagnation, or even re-heating suggests insignificant exhumation and peneplanation; this interval correlates with Late Cretaceous–Eocene red-bed deposition and rifting throughout eastern China. Causes for speculative accelerated cooling after ca. 20Ma may be a far-field effect of the India–Asia collision or Pacific Plate subduction. This study demonstrates that the middle Yangtze Craton was involved in the Cretaceous extension widely occurring in the eastern China continent and suggests that the formation of the Jianghan Basin, a major depocenter in the middle Yangtze Craton, may have initiated at ca. 120Ma.

Paleomagnetism of the 765 Ma Luakela volcanics in Northwest Zambia and implications for Neoproterozoic positions of the Congo Craton

The 765 Ma Luakela volcanics of northwest Zambia are a northeast-trending belt of basaltic to andesitic flows within an essentially undeformed and unmetamorphosed siliciclastic succession that unconformably overlies Neoarchean and Paleoproterozoic rocks of the Congo craton margin. Paleomagnetic analysis of 65 samples from nine sites isolated three magnetization components. Component A, carried mainly by single-domain magnetite, is directed very shallowly to the southeast and northwest, and is interpreted as a primary thermoremanent magnetization. Component B, carried mainly by hematite and oriented mainly shallowly upwards to the southwest, is interpreted to be a secondary overprint, possibly acquired during latest Neoproterozoic (Pan-African) deformation in the Lufilian Arc. A low-stability component C, probably carried by multi-domain magnetite or hematite, or both, is directed very steeply downward, and is similar to Late Paleozoic results from the region. Paleopoles LVA and LVB, corresponding to components A and B, are similar to previous results from mafic rocks of similar age in Tanzania. LVA (40.2°S, 122.0°E) coincides with a reliable pole for the 748 Ma Mbozi syenite-gabbro complex, confirming the position of the Congo craton at 765 to 750 Ma. The secondary LVB pole (38.3°N, 90.9°E), of uncertain age but younger than 765 Ma, coincides with the pole for the *ca*. 795 Ma Gagwe lavas, casting doubt on the assumed primary nature of the Gagwe remanence. Our new data imply that the Congo craton rotated *ca*. 90° clockwise between 750 Ma and the time of B-component acquisition, contrary to previous scenarios in which Congo rotated counter clockwise between *ca*. 800 and 750 Ma.

Sedimentology, structure and SHRIMP zircon provenance of the Woodline Formation, Western Australia: Implications for the tectonic setting of the West Australian Craton during the Paleoproterozoic

The Paleoproterozoic Woodline Formation forms a 50 km long, northeast-trending belt of metasedimentary rocks that unconformably overlies Archean granites and greenstones of the Eastern Goldfields Superterrane near the southeastern margin of the Yilgarn Craton. The Woodline Formation is dominated by mature, quartz-rich sandstone units interbedded with siltstones, and represents the remnants of a broader siliciclastic sequence deposited along a passive margin after c. 1737 Ma. The formation is weakly deformed and metamorphosed to lower greenschist facies, with upright northeast-trending folds resulting from northwest–southeast shortening during the Albany–Fraser Orogeny between 1345 Ma and 1140 Ma. An extensional event at c. 1210 Ma is recorded by the emplacement of the northeast-trending Fraser dyke swarm and formation of the grabens that preserved the Woodline Formation. Three hundred and seventy-one U–Pb SHRIMP detrital zircon analyses from six samples from coarse-grained sandstone units of the Woodline Formation indicate a maximum depositional age of 1737 ± 28 Ma (95% confidence). The age data from all six samples show heterogeneity of age spectra from the Eoarchean to the Paleoproterozoic, with an up-sequence diversification of source terranes. Significant probability maxima at 2687 Ma, 2655 Ma and 2629 Ma are consistent with erosion of the underlying Archean granite–greenstone rocks of the Eastern Goldfields Superterrane and the adjacent terranes of the Yilgarn Craton. In comparison, the Paleoproterozoic zircons with maxima at 2245 Ma, 2034 Ma and 1886 Ma do not match known tectonomagmatic events within the West Australian Craton. Correlation of detrital zircon results from the Woodline Formation with other Paleoproterozoic siliciclastic units around the margin of the Yilgarn reveal a remarkable similarity with the Earaheedy Group, especially with the Paleoproterozoic age peaks indicating a common protosource(s) that is either buried, no longer exists, or is part of an exotic terrane beyond the West Australian Craton.

Geochemistry of the metamorphosed Ordovician Taconian Magmatic Arc, Bronson Hill anticlinorium, western New England

The Bronson Hill anticlinorium (BHA) in western New England is a north- to northeast-trending belt of gneiss domes containing a metamorphosed stratified sequence of Ordovician, Silurian, and Devonian sedimentary and volcanic rocks. These overlie mostly late Ordovician intrusive igneous rocks. The intrusive rocks, exposed in the dome centers, are dominated by felsic gneisses of tonalitic to granitic compositions. They are the exposed parts of a composite batholith that developed along the axis of the Taconian volcanic arc. The Fourmile Gneiss of the Pelham dome, and the Monson Gneiss of the Monson dome, covered in most detail here, are calc-alkaline, dominantly tonalitic to granodioritic gneisses having 61–78% SiO 2. These are divided into two geochemical groups. The most abundant rocks have low Sr (∼50–200 ppm), high Y (∼10–50 ppm), low Al 2O 3, high FeO t, commonly negative Eu anomalies, and have flat to somewhat concave upward MREE and HREE patterns. These rocks were probably derived from melting of plagioclase–pyroxene–amphibole granulites in the deep crust. Rocks of the subordinate high-Sr group are similar in composition to adakites and have high Sr (∼300–600 ppm), low Y (∼1–13 ppm), higher Al 2O 3, lower FeO t, commonly no Eu anomalies, and are strongly depleted in HREE. These rocks were probably derived from a pyroxene–amphibole–garnet ± plagioclase source at higher pressure. Amphibolites are low in abundance in the Fourmile and Monson Gneisses. Most are a low-Nb type (<6 ppm Nb), compositionally similar to typical calc-alkaline island arc basalts. These were probably derived by melting of spinel lherzolite in the mantle wedge under the arc. Gabbroic anorthosite and ultramafic rocks also occur in the Monson dome and appear to be cumulates from magmas similar to the low-Nb amphibolites. A high-Nb (6–12 ppm) amphibolite group was identified and is unique to the Monson Gneiss. These rocks are transitional between calc-alkaline and alkaline basalts, are strongly LREE-enriched (La is 46–440 times chondrite), are enriched in strongly incompatible elements, and two of seven samples have normative nepheline. A broader survey of available analytical data from BHA felsic gneisses shows that gneiss compositions vary from dominantly tonalitic and granodioritic in the southern BHA to dominantly granitic from the Croydon and Mascoma domes (west-central New Hampshire) northward. This variation suggests that the southern part of the arc exposed in the BHA was based on mafic crust, whereas the northern part was based on intermediate to felsic crust. Examination of radiometric dates of Taconian igneous rocks, and biostratigraphic constraints in the Taconic allochthons and in the Taconian foreland, are consistent with a model in which the BHA gneisses are the youngest igneous components of the Taconian arc proper. Collision of this composite arc with Laurentia reached a conclusion in the latest Caradoc or Ashgill of the late Ordovician or in the very earliest Silurian. Younger plutons continuing into the Silurian (e.g., Highlandcroft in part) may be related to magmas generated during post-collision delamination or detachment of the subducted Iapetus oceanic slab.

The Bolu Massif: remnant of a pre-Early Ordovician active margin in the west Pontides, northern Turkey

The scope of this study is to understand better the pre-Early Ordovician history of the west Pontides of northern Turkey by focusing on the best-exposed part of the Bolu Massif, which is located between Bolu and Yedigöller (Seven Lakes). The Palaeozoic rocks of the west Pontides tectonic belt of northern Turkey comprise a transgressive sedimentary sequence known as ‘Palaeozoic of İstanbul.’ In a few areas, the basement of the Palaeozoic sequence is exposed, the largest part of which is the Bolu Massif, which is located in the middle of the west Pontides. The lowermost unit of the Palaeozoic of İstanbul in the Bolu area is the Işığandere Formation, which is made up of fluvial red conglomerates and sandstones of Lower Ordovician age. Three different units are exposed unconformably beneath these continental clastics, forming the Bolu Massif. From the structural base to the top, these are as follows: (1) a high-grade metamorphic unit, known as the Sünnice Group); (2) granitoid intrusions, known as the Bolu Granitoid Complex; and (3) a greenschist meta-volcanic sequence (the Çaşurtepe Formation).The Sünnice Group is the lowest, structurally speaking. It is a southwest–northeast-trending belt of migmatitic basement, consisting of amphibolites and paragneisses cut by small (&lt; 10 m) metagranitic intrusions. The Sünnice Group is tectonically overlain by the Bolu Granitoid Complex and the Çaşurtepe Formation along the Karadere Fault. In the study area the Bolu Granitoid Complex is represented by two distinct, north-northeast–south-southwest-trending intrusions, the Tüllükiriş and Kapikaya plutons. The granitoids are mainly tonalitic and granodioritic in composition, cut by lam-prophyre and aplite dykes and intruded into the Çaşurtepe Formation. The Çaşurtepe Formation is composed mainly of andesitic and minor rhyolitic lavas, along with a meta-ignimbrite sequence.The lavas have geochemical characteristics indicative of eruption in a subduction-related tectonic setting. The geochemistry of the intrusions also suggests emplacement in an arc-type setting. Initial Nd isotope data for the Çaşurtepe Formation indicate derivation from a depleted mantle source, whereas those for the granitoids are consistent with greater degrees of crustal contamination.

Structure of the Reelfoot Rift As Interpreted From 2-D Magnetotelluric Models

Abstract The results of magnetotelluric (MT) surveys reveal structures associated with the Reelfoot rift, including an axial high-resistivity structure that may be related to intrusions in the central part of the rift or to a previously unrecognized horst. The axis of this resistivity high generally follows the central part of the Reelfoot rift, but its orientation is offset several degrees from the enigmatic Blytheville arch. The MT structural high follows the main part of a northeast-trending seismicity belt, but in the New Madrid, Missouri area electrical structures are more complicated. In this northern part of the study area, the strike of resistivity structures determined from the MT data indicate a nearly north-south direction, rather than the N45°E direction indicated for most of the data. If the axial high-resistivity structure represents a horst, then it may form an interior divide between thick shale basins. Faults bounding the proposed horst may be a controlling factor for the northeast trending seismic zone.

Bermuda and Appalachian-Labrador rises: Common non-hotspot processes?

Other than the Corner Rise-New England seamounts and associated White Mountains, most postbreakup intraplate igneous activity and topographic uplift in the western North Atlantic and eastern North America do not readily conform to simple hotspot models. For example, the Bermuda Rise trends normal to its predicted hotspot trace. On continental crust, Cretaceous-Eocene igneous activity is scattered along a northeast-trending belt ∼500-1000 km west of and paralleling the continent-ocean boundary. Corresponding activity in the western Atlantic generated seamounts preferentially clustered in a belt ∼1000 km east of the boundary. The Eocene volcanism on Bermuda is paired with coeval magmatism of the Shenandoah igneous province, and both magmatic belts are associated with northeast-trending topographic bulges—the Appalachian-Labrador Rise to the west and the Bermuda Rise (Eocene?) to the east. The above observations suggest the existence of paired asthenosphere upwelling, paralleling and controlled by the deep thermal contrast across the northeast-trending continental margin. Such convection geometry, apparently fixed to the North American plate rather than to hotspots, is consistent with recent convection models by B. Hager. The additional importance of plate-kinematic reorganizations (causing midplate stress enhancement) is suggested by episodic igneous activity ca. 90-100 Ma and 40-45 Ma.

Stratigraphy of the Wakarusa Limestone Member of the Bern Formation (Wabaunsee Group, Virgilian Stage, Upper Pennsylvanian) of East-Central Kansas

The Wakarusa Limestone Member of the Bern Formation crops out as a prominent east-facing escarpment in a northeast-trending belt across eastcentral Kansas. The limestone is light gray, hard, and fossiliferous. Horn corals are abundant locally. In Greenwood County a small, 18-foot thick algal bank has been identified. The member is interpreted to have been deposited under normal open-marine conditions in the shallow, warm Kansas Sea. The Wakarusa is the upper limestone member of the Bern Formation (Wabaunsee Group, Virgilian Stage, Pennsylvanian System). The Wakarusa crops out in a narrow band across east-central Kansas. The unit was named by J. W. Beede in 1898 for exposures along Wakarusa Creek just south of Auburn in southwestern Shawnee County. One of the best exposures now is along 170 just west of Topeka near the center of section 31, T. 11 S., R. 15 E. (Plate 1). The outcrop band extends diagonally south-southwest from Brown County in the north to Chautauqua County in the south. The best exposures are south of the Kansas River Valley. Because the Wakarusa is a prominent bench-forming limestone across most of the outcrop belt in eastern Kansas, it has received more-than-normal attention from surface mappers and stratigraphers. Following the initial work of Beede (1898), Condra (1927) extended his work south from Nebraska to include the Wakarusa. R. C. Moore followed with a series of reports on the Pennsylvanian in eastern Kansas, which included the Wakarusa (1936, 1937, 1948, 1949, and 1964). The unit has been mentioned in many other publications, theses, and dissertations. There are, however, three theses of particular importance (Owen, 1959; Ross, 1960; and Myers, 1968) and one short publication (Myers and Robbins, 1968). Many of the field guides in eastern Kansas also include a stop or two at an outcrop of the Wakarusa Limestone Member and adjacent beds. This short paper is intended to give This content downloaded from 157.55.39.203 on Sat, 27 Aug 2016 06:13:52 UTC All use subject to http://about.jstor.org/terms 68 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE

A Cambrian island arc in Iapetus: geochronology and geochemistry of the Lake Ambrose volcanic belt, Newfoundland Appalachians

AbstractThe Lake Ambrose volcanic belt (LAVB) outcrops as a 45 km long northeast-trending belt of mafic and felsic volcanic rocks along the eastern side of the Victoria Lake Group in south-central Newfoundland. It comprises roughly equal proportions of mafic pillow basalt and high silica rhyolite, locally interbedded with epiclastic turbidites. Volcanic rocks have been metamorphosed in the greenschist facies and are extensively carbonatized.U-Pb (zircon) dates from rhyolite at two, widely separated localities give identical ages of 513 ± 2 Ma (Upper Cambrian), and this is interpreted as the eruptive age of the volcanic sequence. Primitive arc and low-K tholeiites can be recognized on the basis of major and trace element geochemistry, ranging from LREE-depleted to LREE-enriched. Geochemical variation between mafic volcanic types is interpreted predominantly to reflect contrasts in source characteristics and degree of partial melting; some variation within each geochemical type attributable to fractional crystallization can be recognized. Detailed examination of some samples indicates that the heavy REE and related elements have locally been mobile, probably as a result of carbonate complexing.The LAVB is the oldest well-dated island arc sequence in Newfoundland, and perhaps in the Appalachian–Caledonian Orogen. Its age requires modification of widely held models for the tectonic history of central Newfoundland. It is older than the oldest known ophiolite, demonstrating that arc volcanism was extant before the generation of the oldest known oceanic crust in this part of Iapetus. It further demonstrates that there was a maximum of approximately 30 Ma between the rift-drift transition which initiated Iapetus, and the initiation of subduction. This suggests that the oceanic sequences preserved in Newfoundland represent a series of arcs and back arc basins marginal to the main Iapetus Ocean, and brings into question whether the Appalachian accreted terranes contain any remnants of normal mid-ocean ridge type Iapetan crust.

The Kelian disseminated gold deposit, East Kalimantan, Indonesia

Kelian is one of a number of recently discovered Tertiary volcanic-hosted gold deposits which occur within a 400-km-long, northeast-trending belt in the interior of Kalimantan, the Indonesian part of Borneo. It consists of two main orebodies, namely West and East Prampus, and four smaller mineralized ore zones. These have a combined resource potential of +75 Mt at 1.8 g/t Au, making Kelian the largest known gold deposit in Indonesia. The deposit was discovered in 1976 during follow up work of alluvial gold occurrences within the Kelian River, using traditional exploration techniques of stream sediment, pan concentrate, rock float and outcrop sampling. Subsequent detailed investigations included soil sampling, trenching, deep augering, ground magnetics, induced polarization and 60,000 m of diamond drilling. The geology of the deposit consists of a pile of silicic pyroclastics grading upwards into a series of sediments of Late Eocene age. The sequence was folded and faulted along northerly and northeasterly trends and intruded by a number of subvolcanic andesitic to trachyandesitic bodies in the Early Miocene. Some time after the emplacement of the andesites a hydrothermal system was established. It brought about extensive alteration, mineralization and hydrothermal brecciation. The preserved part of this system is 1 km 2 in area and has a vertical extent of at least 600 m. Limited radiometric age data suggest an age of around 20 Ma for the alteration. Four partly overlapping stages of alteration/mineralization are recognized: 1. Stage I: chlorite+carbonate+sericite+pyrite±epidote alteration in the cores of the larger andesite bodies. 2. Stage IIA: localized fracturing and sericite±quartz deposition. Stage IIB: a major period of polyphasal fracturing and hydrothermal brecciation with associated boiling of hot (290–330°C), dilute (0.5–4.2 eq. wt.% NaCl) and CO 2-rich fluids, and development of replacement/vein alteration of adularia+sericite+quartz+pyrite accompanied by subordinate As-Ag mineralization and possibly some gold mineralization. Progressive loss of CO 2 upon boiling caused a progressive increase in adularia and a corresponding decrease in sericite deposition. Deeper in the system a quartz-sericite-carbonate assemblage was formed without adularia. 3. Stage III: renewed magmatic activity, focused on East Prampus, released volatiles to the hydrothermal system, producing a hot (300–330°C), relatively saline (> 10 wt.% NaCl), CO 2-rich brine, and extensive fracturing and hydrothermal brecciation. This was accompanied by boiling, deposition of various carbonates±quartz±sericite and gold-base metal mineralization; preliminary stable isotope data on the carbonates support the presence of a magmatic component. Mixing of the upwelling hot, saline fluids with descending cooler, relatively low pH, steam-heated fluids has resulted in a complex zonation in carbonate mineralogy and a wide range of fluid salinities and homogenization temperatures. Away from the centre of brecciation and boiling, gold and base metals were deposited in a more passive environment together with carbonate and/or quartz. 4. Stage IV: late-stage widespread influx of relatively cool (<200°C), low pH (3–4), CO 2-rich, steam-heated fluids from the top and margins of the system forming a kaolinite+Fe/Mn carbonate+pyrite ±hematite±cinnabar assemblage. The magmatic-hydrothermal event was followed by uplift and erosion of more than 900 m during the Midddle Late Miocene and basaltic volcanism in the Plio-Pleistocene. A second cycle of erosion, which began in the Pleistocene, has removed most of the young volcanic cover exposing the deposit as known today.

Precambrian paleostress from microcracks and fluid inclusions in the Wolf River batholith of central Wisconsin

A strongly developed set of healed micro-cracks striking approximately northwest-southeast is present at 5 of 6 sample localities within the 1450 ± 20 Ma Wolf River batholith of central Wisconsin. Other minor sets are present but are less well defined. Fluid inclusions in healed microcracks show melting temperatures in 2 groups: 0 °C, indicating nonsaline water, and -3 to -30 °C, suggesting salinities above 4.5%. The in situ trapping temperatures of fluid inclusions, calculated from homogenization temperatures, show a consistent 250-400 °C for all the sets of healed microcracks. Rb-Sr dates of about 1440 Ma on biotites from the Wolf River batholith suggest that the 300 °C blocking temperature of biotite was reached very soon after intrusion and that no reheating event after primary cooling took place in the batholith. The fluid-inclusion data and Rb-Sr date interpreted together suggest that cracks were probably generated and healed during the initial cooling of the pluton, starting at near 400 °C. The preferred orientation of these microcracks implies that the maximum horizontal stress was northwest-southeast for this region about 1400-1450 Ma. A similar orientation of a major joint set in the area also might be correlated with the same event. Strongly developed N20°W fluid-inclusion planes in the Illinois deep borehole UPH-3, about 250 km to the southwest, with similar-age bedrock, may reflect a more distal effect of this same stress filed. The northwest-southeast orientation for the maximum horizontal stress direction during the cooling of the Wolf River batholith is subparallel to the proposed orientation of the paleostress field prior to the emplacement of the Wolf River batholith during an unnamed event that deformed upper Proterozoic quartzites, and it is consistent with a long period of continental collision and accretion, which formed northeast-trending belts along the southern and southeastern margin of the continent. The orientation of the regional stresses may have remained fairly constant during much of the period from Penokean to the start of the Keweenawan, when the maximum regional horizontal stress changed from a northwesterly orientation to a northeasterly one. The data show that even in periods of relative tectonic quiescence, such as during intrusion of the Wolf River batholith, micro-cracks may provide a record of paleostress orientations.

Fluid inclusions and rare earth element geochemistry of fluorite from south-central Idaho

Epithermal fluorite mineralization of Tertiary age occurs in a northeast-trending belt within the Challis 2 degrees quadrangle of south-central Idaho. Fluorite was collected from the Meyers Cove area and the Bayhorse, Yankee Fork, and Stanley mining districts. Host rocks include the Eocene Challis Volcanics, the Cretaceous Idaho batholith, and the Ordovician(?) Bayhorse Dolomite. Mineralization is structurally controlled and is present as fault and fracture fillings and breccia cements. Argillization and silicification of the wall rocks is common and locally intense.Microthermometric measurements on nearly 400 two-phase (H 2 O + vapor), water-rich inclusions with low vapor to liquid ratios yielded mean homogenization temperatures of 160 degrees C for the Meyers Cove area, 150 degrees C for the Stanley district, and 130 degrees C for the Bayhorse and the Yankee Fork districts. Salinities range from 0 to 1.4 equiv wt percent NaCl.The rare earth element content of the fluorite is quite variable, ranging from 2.6 to 130 ppm, and Ce/Yb ratios reflect the dominant trend of light rare earth element enrichment. Fluorite from the Meyers Cove area and the Bayhorse and Stanley districts have similar Tb/La ratios, indicating crystallization at about the same time. Fluorite from the Yankee Fork district has a lower mean Tb/La ratio which is indicative of earlier crystallization. Chondrite-normalized plots and rare earth element ratios reveal persistent negative Ce anomalies, indicating high oxygen fugacities at the source area. Positive Eu anomalies indicate the release of Eu (super +2) during feldspar destruction, oxidation to Eu (super +3) at the deposition site, and subsequent incorporation in the fluorite. Samples with negative Eu anomalies indicate the presence of divalent Eu.The fluid inclusion, geochemical, geologic, geochronologic, and stable isotope data support deposition in Tertiary hydrothermal systems dominated by fluids of meteoric origin. The most important depositional mechanism is believed to be an increase in the pH of the ore-bearing fluids upon interaction with the wall rocks.

Neotectonic movement, earthquakes and stress state in the eastern United States

A regional pattern of crustal movement across the eastern United States is suggested by relations found by investigations into the cause of earthquakes. The pattern consists of zones of fractures and vertical movement. Most of the seismic source areas appear to be related to northwest-trending fracture zones, that commonly have apparent righ-lateral strike-slip offset, where they cross broad northeast-trending belts of vertical movement. The fracture zones also appear to control the positions of local northwest-trending basins, embayments, on the Atlantic and Gulf Coastal plains. These embayments seem to be relatively subsiding at present and have earthquakes associated with them; intervening arches have very few earthquakes. Some earthquakes are also spatially related to north-trending extensional fault zones that commonly lie north of northwest-trending fracture zones. A slight northward tilt of the entire region and possible lingering effects of glacial rebound in the northeast show no relation with seismicity. Both the broad belts of vertical movement and embayments are apparently those initiated by the Late Cretaceous as the North Atlantic basin widened, and the northwest-trending fracture zones can be directly or indirectly related to the offshore transform fracture zones. The interplate movements in the eastern United States are thus directly related to those in the North Atlantic basin. The fracture pattern is also similar to that of the western United States, but the movements less. These tectonic movements in the eastern United States can be accounted for by a maximum horizontal compressive stress oriented approximately north-northwest-south-southeast to north-south and an extension oriented approximately east-west.

Deformation, geochemistry, and origin of massive sulfide deposits, Gossan Lead District, Virginia

The Gossan Lead district is a 28-km-long, northeast-trending belt of discontinuous massive sulfide deposits in the Blue Ridge province of southwestern Virginia. The deposits, hosted by the Ashe Formation of late Proterozoic age, consist of strata-bound lenses and layers of massive pyrrhotite, minor chalcopyrite, sphalerite, and pyrite, and rare arsenopyrite and galena. Deposits were mined principally in the Iron Ridge and Betty Baker segments, respectively, at the southwestern and northeastern ends of the belt. Detailed mapping of the Gossan Howard, Huey, and Bumbarger pits in the Iron Ridge segment indicates that the deposits occur at one horizon and have been variously folded and brecciated after sulfide deposition. The Gossan Howard consists of a single, gently dipping lens of sulfide. The Huey deposit is complexly folded and locally contains tectonically thickened ore. The Bumbarger deposit is a lens as much as 40 m thick--the thickest known in the district. This deposit contains abundant coarse breccia fragments of wall rock around which the massive sulfide has flowed (during deformation and metamorphism), probably thickening the original deposit significantly. In the northeastern part of the district, drill holes intersect several sulfide layers that possibly are structurally repeated.The Ashe Formation in the district is a sequence of metasedimentary rocks and local conformable lenses of amphibolite and actinolite-chlorite schist. The metasedimentary rocks include metapelite, quartz-feldspar granofels (metagraywacke), and minor quartzite and carbonaceous schist, and are interpreted as marine turbidites. The amphibolites and other mafic rocks have chemical compositions similar to low Ti tholeiitic basalt, with a high Y/Nb (>10) and high average contents of Co (40 ppm), Cr (403 ppm), Ni (211 ppm), and V (247 ppm). Immobile trace element signatures (Ti-Y-Zr; Th-Hf-Ta; Ti-Cr) suggest a magmatic affinity with midocean ridge basalt (MORB); rare earth elements (REE) have low abundance levels (10X-15X chondrite), broadly flat patterns [(La/Yb) N = 0.7-1.1], and a slight depletion in the light elements similar to midocean ridge basalts. An amphibolite from a much higher stratigraphic level, south of the district, differs significantly from the mafic rocks closer to the sulfide zone in having the chemical signature of a transitional, slightly alkalic tholeiite with high TiO 2 (3.87 wt %), Fe 2 O 3 (16.4 wt %), and P 2 O 5 (0.56 wt %), low Y/Nb (3.3), and a highly fractionated rare earth element distribution [(La/Yb) N = 3.9] similar to continental basalt.Some silicate wall rocks of the deposits are mineralogically and chemically unusual, and differ substantially from the clastic metasediments of the Ashe Formation. Such rocks are composed mainly or wholly of plagioclase feldspar, biotite, chlorite, muscovite, or spessartine-rich garnet. The unusual lithologies form local strata-bound lenses in the footwall and/or hanging wall of the deposits, typically within 10 m of massive sulfide. The plagioclase rocks (3.4-7.6 wt % Na 2 O) consist largely of granoblastic albite-oligoclase (Ab (sub 78-90) ) with minor quartz and biotite; rare earth elements are highly fractionated [(La/Yb) N = 6.8-7.1] and their patterns resemble those of the quartz-feldspar granofels (metagraywacke) from the district.The biotite schists, locally monomineralic, have FeO/(FeO + MgO) = 0.5 and contain high phosphorus (1 wt % P 2 O 5 ) and fluorine (0.5 wt % F), present in fluorapatite. The chlorite schist is essentially all ripidolite; rare earth elements are highly fractionated, and have a large negative Eu anomaly. The spessartine-rich rocks (6.3-8.9 wt % MnO) are in places interlayered with the other unusual wall rocks and consist of abundant Mn-rich garnet (Sp 50 Al 23 Gr 19 Py 8 ) and minor quartz, plagioclase, pyrrhotite, and biotite. The distinctive mineralogy and chemistry of these rocks suggest that they represent metamorphosed alteration zones and/or intermixed chemical and clastic sediments.The sulfide deposits are interpreted as syngenetic in origin but modified in form by deformation which accompanied metamorphism. The great length of the mineralized district parallel to the regional strike and the flyschoid (turbidite) nature of the host rocks suggest that sedimentation and initial sulfide deposition took place in a deep, elongate marine basin or graben overlying a crustal rift zone. A rift underlying the sedimentary pile is consistent with the occurrence of mafic metavolcanic rocks of midocean ridge basalt affinity and could also have served as the feeder system for a line of hydrothermal vents on the sea floor that generated the sulfide deposits.

Contrasting styles of Sn-Nb-Ta-Zn mineralization in Nigeria

Two distinct and economically important types of primary Sn-Nb-Ta-Zn mineralization occur in Nigeria.The early stage is related to Palaeozoic pegmatites which occur sporadically in a 400 km long north-east trending belt. Mineralogically the pegmatites consist of quartz, potash feldspar, albitized mica and are characterized by extensive development of tourmaline. Economically they are important for minerals of the columbo-tantalite series.The later stage of mineralization is found in granites of the Mesozoic ring complexes. Tantalum-bearing columbite, accompanied by cassiterite, and rare earth minerals have been disseminated in the cupola zones of biotite granites during an albitization process. A similar albitization process in the arfvedsonite granites has led to the dissemination of pyrochlore, thorite, monazite and xenotime. Mineralized veins, which are limited in occurrence to the biotite granites, form four major types which contain either an oxide series of minerals dominated by cassiterite and wolframite or a series of sulphides dominated by sphalerite.Fluid inclusion studies related to the Younger Granite mineralization show a gradual evolution of fluids from highly saline, at around 500°C, to dilute fluids at 320°C or below. It is from these later dilute fluids that the sulphble ore assemblage is deposited.The contrasting styles of Palaeozoic pegmatite and Younger Granite mineralization are related to granites of differing chemistry whose tectonic setting may also be different.

Cretaceous magnetizations in northwestern Nevada and tectonic implications

Paleozoic and Mesozoic arc-related rocks of the Black Rock Desert region of northwestern Nevada have acquired a very stable postfolding secondary magnetization, probably in Late Cretaceous time. The remagnetization, which appears to be of regional extent, probably accompanied emplacement of granitic plutons in a northeast-trending belt across northwestern Nevada near the end of Mesozoic time. The region has experienced no appreciable tectonic rotation since that time.