Geochemistry and Geochronology of the Cretaceous Igneous Rocks in the Mogpo Area
3
Citation
0
Reference
20
Related Paper
Citation Trend
Abstract:
Igneous rocks of Mogpo area composed of mainly Cretaceous volcanic and granitic rocks, especially volcanic rocks included with some glassy rock. The glassy rock corresponded to pitchstone with waxy and resinous luster because of absorbed water from neighboring hydrous rock formations. The pitchstone occur as dike in Gajido, Momilhang and Gohado of Mogpo area, and as small stock in Geumdangdo area. The Cretaceous igneous rocks plot on trend of calc-alkaline and corresponded to sub-alkalic rock series, but pitchstone show different trend from these igneous rocks, and are a series of differentiated products from cogenetic magma. In minor elements, igneous activity of Mogpo area start from volcanic activity, and continued on intrusive rocks at end of the Cretaceous. Pitchstone with relatively high differentiation intrued into hydrous rock formation at early Tertiary (Eocene). In chondrite normalized REE pattern, REE pattern are similar to micrographic granite-pitchstone and rhyolite-tuff, respectively. The former enriched than the latter in LREE, and have high Eu(-) anomaly. Parental magma type of these igneous rocks correspond to mantle fractionation+Syn-collision(volcanics). VAG field in Pearce diagram, and NCA field in volcanic arc maturity. Isotopic age of igneous rocks in the Mogpo area are 82.8∼86.4 Ma, 61.6∼81.4 Ma, 71.8∼75 Ma and 81.2∼86.9 Ma and 41.4∼42.3 Ma in tuff, rhyolite, porphyries and pink feldspar granite, respectively. These isotopic and geochemical data suggest the Cretaceous igneous rocks of Mogpo area were intruded and emplaced at two orogeny associated with igneous activity (Bulkuksa and Youngil disturbance). Pitchstone intrued into hydrous rock formations at late Eocene (in MogPo area) and late Cretaceous (in Geumdangdo area). and concluded pitchstones of both area have similar geological environment that rhyolitic magma intruded into neighbouring hydrous rock formation.Cite
Summary Thick subaerial volcanic sequences of probable mid Triassic age rest unconformably on Permo-Carboniferous limestones in the ‘Phyllite Series’ within the Gavrovo-Tripolitsa zone of the external Hellenide nappes. The volcanic rocks are varied in character. Pyroclastic rocks (often reddened) predominate, but minor basalt or andesite and rhyolitic hypabyssal intrusions and flows are also found. The rocks have experienced low-grade metamorphism and the only relict primary minerals are pyroxene and rare plagioclase in basic rocks and potash feldspar in acid rocks. The most important metamorphic phases are albite, chlorite, potassium mica, epidote, hematite, quartz and pumpellyite. Twenty-seven whole-rock major-element analyses suggest that there has been some exchange of Na 2 O and K 2 O for CaO during metamorphism, but that other elements have been relatively stable. A genetic interpretation of the rocks is attempted using published geochemical discriminator diagrams. The basic rocks are tholeiitic in major element chemistry and pyroxene composition. Trace element (Hf, Ta, Th, Ce, Yb) distribution suggests magma generation at a destructive plate margin.
Pyroxene
Cite
Citations (6)
Cenozoic volcanic rocks outcrop in the central portion of the Loei–Phetchabun volcanic belt in central Thailand in the Lop Buri area. The volcanic rocks range in composition from basalt to high-silica rhyolite. In general, the volcanic rocks decrease in age from south to north. The oldest rocks studied are 55–57 Ma rhyolites that are isotopically and geochemically distinct from younger (13–24 Ma) rhyolites that occur farther north. Intermediate rocks (andesite and dacite) are less voluminous than rhyolite. Basalt occurs in the central and northern parts of the area and ranges in composition from olivine tholeiites to nepheline normative alkali basalts. The isotopic, major, and trace element compositions of the andesites, dacites, and younger rhyolites are consistent with an origin for these rocks by variable degrees of partial melting of metabasaltic crustal rocks, themselves derived from a depleted mantle source at approximately 530 ± 100 Ma. The apparent extent of partial melting of metabasalt increases from rhyolite to andesite. The isotopic and trace element systematics of the basalts are consistent with a refertilized depleted mantle source with characteristics of a mixture of normal mid-ocean ridge basalt source mantle and enriched mantle II type mantle.
Silicic
Alkali basalt
Dacite
Cite
Citations (18)
Felsic
Silicic
Fractional crystallization (geology)
Cite
Citations (24)
Petrogenesis
Sill
Trachyte
Felsic
Large igneous province
Cite
Citations (8)
The Wild Bight Group, part of the Newfoundland Dunnage Zone or Central Mobile Belt, is a thick (probably more than 8 km.) sequence of dominantly epiclastic (~75%) and lesser volcanic (~25%) rocks which outcrops in and to the south of central Notre Dame Bay. The base of the group is not exposed. It passes conformably upward into fossiliferous shale with a Caradocian (Middle Ordovician) graptolite fauna. On this basis, it is considered as Early to Middle Ordovician in age. -- Volcanic rocks occur throughout the stratigraphic section of the Wild Bight Group. Eleven separate volcanic sequences are identified and sampled, most of which comprise dominantly or wholly mafic pillow lava. Less commonly, massive basalt or pillow breccia and associated mafic pyroclastic rocks are the dominant lithology. Felsic volcanic rocks occur in five of these sequences and volcanogenic sulphide deposits or prospects in four of them. The entire assemblage is intruded by fine to medium grained mafic sills and, less commonly, dykes, which are interpreted on field and geochemical evidence to be subvolcanic. -- The mafic volcanic rocks exhibit a greenschist facies metamorophic assemblage of chlorite - albite - quartz - epidote ± (actinolite, sphene, magnetite and calcite). The only primary mineral remaining is clinopyroxene. Comparison of secondary mineral chemistry with ancient and modern oceanic rocks and with experimental results are consistent with metamorphism at temperatures in the range 200°C to 280°C and low water/rock ratios. The local presence of secondary amphibole is interpreted to reflect slightly higher temperatures. -- Major and trace element whole rock analyses of volcanic and subvolcanic rocks, as well as clinopyroxene mineral chemistry, reveal a complex geochemical association. Two broad paleotectonic environments can be identified using high field strength elements; volcanic rocks in the lower and middle parts of the group have a clear island arc geochemical signature (negative Ta and Nb and positive Th with respect to La on a chondrite-normalized basis) whereas those in the upper parts of the group generally do not. However, basalts of both affinities occur together in one sequence at the top of the group, suggesting that magmatism of island arc and non-arc affinity overlapped in time. -- Within these two environments, further variations can be recognized. Rocks with an island arc geochemical signature include both LREE-depleted and LREE-enriched island arc tholeiites as well as a group of very incompatible element-depleted tholeiites which are interpreted to represent partial melting of refractory sources. Rocks lacking the island arc geochemical signature range from slightly LREE-enriched basalts to alkali basalts and include a group of basalts that have geochemical characteristics transitional between the two end members. Mafic subvolcanic rocks represent all of these eruptive types. -- Felsic volcanic rocks are low-K, high-SiO₂ rhyolite. They occur only in the central stratigraphic parts of the group and are associated with mafic rocks of island arc affinity. -- Petrogenetic modelling of the Wild Bight Group volcanic rocks, using a comprehensive suite of Nd isotope analyses on selected whole rocks, allows further interpretation of their origin. Rocks of island arc affinity, for the most part, have epsilon Nd in the range -1.2 to +4.8, indicating the involvement of enriched mantle sources in the magmas. Negative Nb and Ta and positive Th anomalies with respect to the LREE indicate that this component was most likely a continental crustal source from the subducting slab. The island arc tholeiites can be modelled as resulting from mixing of this crustal source and normal depleted mantle followed by varying amounts of partial melting. However, the melting history of the depleted tholeiites must be more complex, as suggested by a strong negative correlation between epsilon Nd and both Sm/Nd and atomic Mg/(Mg+Fe) (Mg#). All rocks that lack the island arc geochemical signature have epsilonNd in the range +4 to +7 and are interpreted to have resulted from varying degrees of partial melting of an ocean island basalt-type source, locally mixed with normal depleted mantle.
[Please see thesis for remainder of abstract.]
Greenschist
Felsic
Pillow lava
Sill
Cite
Citations (5)
Fractional crystallization (geology)
Basaltic andesite
Petrogenesis
Lithophile
Cite
Citations (59)
Volcanic rocks and associated dikes have been exposed in Wang Nam Khiao area, Nakhon Ratchasima Province, northeastern Thailand where complex tectonic setting was reported. These volcanic rocks are classified as rhyolite, dacite, and andesite while dikes are also characterized by andesitic composition. These dikes clearly cut into the volcanic rocks and Late Permian hornblende granite in the adjacent area. Rhyolite and dacite are composed of abundant plagioclase and quartz whereas andesite and andesitic dike contain mainly plagioclase and hornblende with minor quartz. The volcanic rocks typically show plagioclase and hornblende phenocrysts embedded in fine-grained quartz and glass groundmass whereas dike rocks contain less glass matrix with more albitic laths. P-T conditions of crystallization are estimated, on the basis of Al-in-hornblende geobarometry and hornblende geothermometry, at about 4.5 – 5.5 kbar, 861 – 927 oC and 4.8 – 5.5 kbar, 873 – 890 oC for the magma intrusions that fed volcanic rocks and andesitic dikes, respectively. Whole-rock geochemistry indicates that these rock suites are related to calc-alkaline hydrous magma. The enriched LILE (e.g. Rb, Sr) and depleted HFSE (e.g. Nb, Ce, Ti), with similar REE patterns indicate arc magmatism. The results of this study are comparable to the continental arc magmatism along the Loei Fold Belt as a consequence of the Late Permian Palaeo-Tethys subducted beneath Indochina Terrane.
Volcanic arc
Island arc
Cite
Citations (6)
Felsic
Petrogenesis
Fractional crystallization (geology)
Anatexis
Cite
Citations (34)
Silicic
Felsic
Fractional crystallization (geology)
Cite
Citations (10)