排序方式： 共有2条查询结果，搜索用时 0 毫秒
1
1.
Archean tonalitic-granodioritic orthogneisses bordering theSkaergaard Intrusion contain widespread boudins and lenses ofgarnet-biotite schist, quartzite, amphibolite, and ultramaficrocks. These rocks are similar to and locally gradational withnarrow intact supracrustal belts in the region. We correlateearliest isoclinal folds in supracrustal belt rocks and in earliesttonalitic-trondhjemitic-granitic (TTG1) orthogneisses with regionallydeveloped (D2) deformation. We also correlate strong foliation(Ssp1) in the supracrustal rocks and banding (Sbgn1) in earliestorthogneisses with D2 deformation which followed and overlappedearliest M1 metamorphism. Ssp1 foliation is in part axial planarwith D2 isoclinal folds transposing compositional and subparallelmetamorphic banding in the hinge areas. Ssp1 assemblages inmetapelites consist of folia of coexisting sillimanite-biotite-quartzand correspond roughly to metamorphism at the second sillimaniteisograd. We correlate syntectonic emplacement of a later generationof orthogneisses (TTG2) with strong D3 shearing-cataclasis associatedwith tectonic intercalation of supracrustal rocks and earliestorthogneisses. The latest metamorphic assemblages (M2) consistof granoblastic and porphyroblastic minerals that overprintSsp1 (M1) foliation and D3 fabrics. These assemblages formedduring largely static regional metamorphism about 2900 Ma agoand are locally aligned with fabric elements of D4 folding. Temperatures during M2 metamorphism equalled or exceeded thestability of biotite-sillimanite-quartz in metapelites and chlorite-orthopyroxene-olivine-spinel-hornblendein ultramafic rocks. Fe-Mg biotite-garnet exchange, and thepressure-dependent garnet-plagioclase-sillimanite-quartz equilibriumassemblage in metapelites yield temperature and pressure estimatesfor M2 metamorphism of 650–700?C and 3–4 kb. Thesedata suggest that M2 assemblages formed as results of dehydrationreactions at water partial pressures that were less than thetotal pressure. The temperature-dependent equilibrium assemblagechlorite-orthopyroxene-olivine-spinel-vapor (+hornblende), adjustedfor observed phase compositions, is consistent with the Fe-Mgbiotitegarnet exchange geothermometer. Rare-earth element, Rb-Sr and Pb-Pb isotopic, and other compositionalcharacteristics of the orthogneisses are generally consistentwith a multiple stage magmatic origin of their protoliths. OlderTTG1 orthogneisses have compositions generally consistent withformation of the magmas parental to their protoliths by partialmelting of garnetiferous source rocks such as eclogite, or lowercrust. Younger TTG2 orthogneisses have compositions that areconsistent with their formation as water-saturated second meltsin equilibrium with a hornblende-rich residuum. Their formationoccurred within a few 107 y after crustal emplacement of TTG1orthogneisses. The source of water for the formation of later(TTG2) melts may have been M2 dehydration reactions deeper withinthe supracrustal pile. 相似文献
2.
Miocene volcanism in the Blue Mountains province of centralOregon produced diverse basaltic rocks. One set of these, thePicture Gorge Formation of the Columbia River Basalt Group,is well known. Others (Bowman Dam, Bear Creek, and Slide Creekflows) are relatively poorly known. Only the Picture Gorge flowsin the center of the province are typical continental floodbasalts. Basaltic rocks with calc-alkaline affinities (evolvedBear Creek flows, Slide Creek basalts) are found to the westand east. Basal Bear Creek flows closely resemble MORB and islandarc tholeiites, despite having erupted on a continental plate.Bowman Dam (formerly ‘Prineville’) basalts are richin K2O, P2O5, and Ba, and poor in Ni, Co, and Cr. Some of thesefeatures may reflect mantle metasomatism or crustal contamination.Thus, several currently debated theories of basalt petrogenesiscan be tested by studying flows erupted during Miocene timesin this relatively small area. Very few, if any, of these basaltic rocks represent primarymantle-derived magmas. Inferred depths of the pre-eruption magmareservoirs in which they were fractionated are shallow in thecenter of the province, deeper to the east and west. The E-W axis of Miocene calc-alkaline rocks in the Blue Mountainsprovince existed simultaneously with a N-S axis of calc-alkalineactivity in the Cascades that paralleled an offshore subductionzone. Current ideas of relationships among tectonic settingsand the compositions of volcanic rocks cannot easily be madeto conform with these facts. 相似文献
1