Rocks Of Different Lithologies Research Articles

The concept of model-temperature in oxygen isotope geochemistry: An example of a single outcrop from the Rayner Complex (Enderby Land, East Antarctica)

Oxygen isotope distribution among rocks of different lithologies from one outcrop of the polymetamorphic Rayner Complex (East Antarctica) was used to estimate the “model temperatures” ( T m ), which correspond to the change of fluid behavior from the large-scale migration to small-scale exchange (open-system vs. closed-system) during waning stages of regional metamorphism T m are deduced assuming 1. (1) isotopically homogeneous fluid before isolation of a sample-scale system from its surroundings and 2. (2) mass-balance control of the fluid isotopic composition after the isolation. An estimated T m of about 720°C fits data on cationic geothermometers and equals the peak conditions of the “Major Rayner” event. Conventional oxygen geothermometry, applied to the same samples, gives equilibrium temperatures that are generally lower (down to 565°C), though the local-scale oxygen transport among minerals mostly caused disequilibrium in isotope distribution.

Chronology of tectonic events in the crystalline core of the Himalaya, langtang National Park, central Nepal

The Main Central Thrust (MCT) is an important intracontinental subduction zone that accommodated a significant proportion of shortening between India and Asia during Tertiary Himalayan orogenesis. Argon 40/argon 39 geochronology indicates at least two distinct periods of thrust movement on the MCT in the Langtang National Park region of central Nepal. Ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40Ar/39Ar dating of muscovites to have occurred sometime before 5.8 Ma. A later period of brittle deformation, resulting in the juxtaposition of rocks of different lithology within the MCT zone, occurred at approximately 2.3 Ma, based on 40Ar/39Ar dating of neoblastic muscovites from the brittle fault zones. The hanging wall of the MCT contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the Greater Himalayan sequence. Argon 40/argon 39 cooling ages of muscovite and biotite from the gneisses range from 4.6 to 9.7 Ma. These dates contrast with previously obtained 16–21 Ma U‐Pb monazite and zircon ages for metasedimentary rocks from the same structural levels [Parrish et al., 1992], indicating relatively slow cooling over the early to middle Miocene interval for much of the MCT hanging wall. However, one 19.3 Ma biotite 40Ar/39Ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than U‐Pb monazite ages for nearby anatexites, possibly suggesting rapid cooling of the uppermost Greater Himalayan sequence by tectonic denudation associated with the structurally higher South Tibetan detachment system. The general consistency of 40Ar/39Ar ages throughout the 11‐km‐thick Greater Himalayan sequence suggests rapid cooling in late Miocene time, probably due to an increase in erosion rate related to ramping on the structurally lower Main Boundary Thrust. An alternative possibility would be massive hydrothermal resetting in late Miocene time of the entire sequence.

Coal-Cleat Domains and Domain Boundaries in the Allegheny Plateau of West Virginia

Regional face cleats cutting Pennsylvanian and Permian coal seams in the Allegheny Plateau of West Virginia can be divided into domains separated by boundaries that are sharply defined along most of their lengths. Domains and domain boundaries are established based on cleat trends, number of regional cleat sets, and relative chronology of cleat-set development. Regional cleats of each set show a common trend, and abutting relationships between multiple sets within a domain describe a progressive history of changing stress fields and cleat development. Boundaries dividing the in-situ directions of horizontal principal stress are coincident with cleat domain boundaries suggesting a common and persistent controlling factor. Uniform trends of face cleats within domains and ab upt changes in cleat signature across domain boundaries can be spatially related to regional basement structures and a depositional hinge line within the coal-bearing and underlying Mississippian rocks. In addition, fold structures in coal and underlying sedimentary rocks across the Allegheny Plateau commonly terminate or change trend abruptly at joint domain boundaries. In some cases, regions of common fold trends in coal bearing rocks are contained within specific domains. Face-cleat trends commonly differ from joint trends in rocks immediately bounding coal seams. However, one domain boundary in coal can be traced into Mississippian rocks through the unconformity at the base of the Pennsylvanian section. In this case, Mississippian and Pennsylvanian joint trends differ. It follows tha joint domain boundaries can extend downward through rocks of different lithologies, as well as coincide with conformable and unconformable stratigraphic boundaries.

New Cambrian and Ordovician paleomagnetic poles for the North China Block and their paleogeographic implications

Multicomponent magnetization has been revealed in several Cambrian and Ordovician sedimentary units from localities in Hebei and Shandong provinces of the North China Block (NCB). A well‐defined component with lower unblocking temperature is characterized by northerly declination and moderate to steep inclination with both polarities, and postdates the Late Cretaceous folding. The pole position derived from the overprinted directions is close to both the Cretaceous pole for the NCB as well as the present North Pole. The overprint is not a simple thermal remanent magnetization, but is probably a chemical remanent magnetization imposed at moderately elevated temperature during the major episode of regional deformation in the Late Mesozoic. The higher unblocking temperature characteristic components pass reversal, baked contact, and fold tests, and are internally consistent with rocks of different lithologies from widely distributed localities. Moreover, the data exhibit an overall similarity in the magnetic polarity pattern in the stratigraphic sequence at sampled localities. Hence the high unblocking temperature characteristic component was probably acquired at or near the time of deposition. The Cambrian and Ordovician paleopoles are somewhat different from the other coeval poles for the NCB, which are based on much less data and do not pass as many reliability tests, but all imply that the NCB was in the equatorial zone in the Early Paleozoic.

A study of the organic matter in Tunisian phosphates series: Relevance to phosphorite genesis in the Gafsa Basin (Tunisia)

In the Gafsa Basin, Tunisia, peloidal fracolite is interbedded with shales, limestones and rocks of intermediate lithology. The rocks were deposited in an epicontinental sea and have been buried no more than 500 m. The organic matter they contain is, therefore, thermally immature and, in all units, derived mainly from marine plankton, particularly Dinoflagellates. The organic matter in the phosphorite units is composed predominantly of humic acids (HA) which are essentially located within the pellets. These pellets are surrounded by an outer matrix in which the organic matter is similar to the kerogen comprised in the shally strata alternating with the phosphorite and limestone beds. During weathering, the content of the organic matter assocciated with the matrix decreases more rapidly than that of the organic matter entrapped within the pellets. Geochemical and palynological study of the organic matter in interbedded rocks of different lithology (shale, limestone, phosphorite) suggest that the shales were deposited in a reducing environment, the limestones in one that was oxic, whilst the phosphorites were formed in a redox environment inermediate between the two. The interpretation of the data presented in this paper is that the genesis of shales, phosphate and some carbonate beds (excluding coquinas) did not result from the deposition of initially sharply differentiated sediments but resulted from differences in diagenesis of sediments initially containing planktonic organic matter associated with clay and carbonate minerals.

Strain refraction in layered systems

Strain refraction across competence contrasts is presented as a simple model consisting of two components, a homogeneous strain and a heterogeneous simple shear. For Newtonian materials, the ratio of the layer-parallel simple shear component in adjacent layers is the inverse of their viscosity ratio. Strong changes in ellipsoid size, shape and orientation are predicted across viscosity contrasts. The geological implications of strain refraction theory are considered within the context of the ‘cleavage/strain debate’. The particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies (and kinematic histories) should appear to be subparallel to the XY planes of measured strain ellipsoids. Competent rocks should develop dominantly layer-orthogonal strain, and incompetent layers shear-dominated deformation. A variety of structural features ranging from cleavage refraction, changing lineation orientations, folds transected by cleavage, changes from coaxial to non-coaxial deformation, and ramp-flat fault geometry may be the result of stress and strain refraction in rocks.

The Piedmont landscape of Maryland: A new look at an old problem

AbstractThe Piedmont upland of Maryland has been variously interpreted as a peneplain, a series of peneplains, a surface of marine planation, and a landscape in dynamic equilibrium. These different perspectives of landform evolution are related to different scales of time and space. Both equilibrium and episodic erosion features can be recognized in the modern landscape. An equilibrium condition is suggested by adjustment of first and second order streams to rock structure and lithology, entrenchment of some streams against gneiss domes, altitudinal zonation of rock types around gneiss domes, correlation of lithology with overburden thickness on uplands, decreasing overburden thickness on uplands related to decreasing degree of metamorphism of crystalline rocks, and correlation of secondary mineral assemblages with subsurface drainage and slope. The long‐term episodic character of erosion is suggested by clastic wedges on the adjacent Coastal Plain, an upland of low relief that truncates non‐carbonate rocks of different lithologies, isovolumetric chemical weathering of alumino‐silicate rocks, clastic deposition in marble valleys, and weathering profile truncation by modern drainage.The Maryland Piedmont may have been an area of positive relief subject to subaerial erosion since Triassic and possibly Permian time. The upland surface preserved in the eastern Piedmont developed by the Late Cretaceous. In the interval from the Late Cretaceous to the Late Miocene, low input of terrigenous sediments to the Coastal Plain, dominance of marine sedimentation, and spotty evidence of saprolite formation on crystalline rocks, suggest that the Maryland Piedmont was an area of low relief undergoing intense weathering. Incised valleys were formed during a cycle of erosion probably initiated in the Late Miocene and extensive colluvial sediments were deposited on hillslopes by periglacial processes during the Pleistocene.