Petrogenetic Relationships of Volcanic and Intrusive Rocks of the Andes
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There are three distinct zones of recent volcanic activity in the Andean Cordillera: a northern zone in South Colombia and Ecuador, a central zone in South Peru and North Chile, and a southern zone in South Chile (Figure 1). Cenozoic intrusive rocks occur between these areas, and on the coastal margin of the active zones. It is therefore natural to suppose that the igneous processes that take place along the Andean plate margin have simultaneous volcanic and plutonic components and, consequently, that the two kinds of magmatic activity are interrelated. This supposition is expressed in countless diagrams and cross-sections purporting to illustrate destructive plate margin processes.Keywords:
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Investigation of igneous rocks in Huanghua depression, North China, from magnetic derivative methods
The igneous rock study plays an important role in understanding the tectonic evolution of sedimentary basins. The distribution of igneous rocks is one of the basic aspects in the igneous rock study. Based on the magnetic susceptibility contrast of igneous and other rocks, magnetic methods are usually used in the igneous rock investigation. In this paper, we delineated the distribution of igneous rocks in Huanghua depression based on the reduction to pole (RTP) magnetic anomaly and its derivative processing results. The main methods used to enhance the anomaly character of igneous rocks include total horizontal derivative, analytic signal modules and the tilt angle. Based on the RTP anomaly and the derivative results, three types of igneous rock distribution areas are predicted in Huanghua depression. The large scale igneous rocks are mainly distributed in the north part of Huanghua depression. The string bead-shaped igneous rocks are mainly located in the north and southwest parts. Some craters are located in the middle-south part. The distribution of igneous rocks is controlled by the deep buried fault along the coastal boundary and the associated tertiary faults. Our study verified the validity of the magnetic derivative methods for the planar distribution study of igneous rocks. The study results of Huanghua depression are the basis of the seismic interpretation of igneous rocks and the tectonic study in this area.
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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.
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Ultramafic rock
Felsic
Aeromagnetic survey
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Ultramafic rock
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A geological survey has been performed at Arjosari geothermal area, Pacitan, East Java. Hot springs, alterationrock, and other surface manifestations indicate possible geothermal energy potential of the area. Geologically four primary rocks are found in the area. Those are sandstone and volcanic breccia of Mandalika Formation, volcanic rocks of Arjosari Formation, igneous rocks on the bottom of the area and alluvium deposit. Regionally many igneous rocks are exposed as intrusive genetic. The volcanic and igneous rocks found in that area are faulted and have fractures making conductive and convective heat transfer happened to cause the generation of hot springs, alterationrock, and other surface manifestation of geothermal characteristics. With the existence of various volcanic and igneous rocks, the surface manifestation of hot springs and alteration rocks, it can be indicated the existence of geothermal resoucein the area.
Breccia
Geothermal exploration
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