Topic
Porphyritic
About: Porphyritic is a research topic. Over the lifetime, 2667 publications have been published within this topic receiving 55550 citations. The topic is also known as: porphyric rock.
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TL;DR: More than half of the known porphyry copper deposits, defined in terms of contained copper metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene to Pliocene as discussed by the authors.
Abstract: More than half of the 25 largest known porphyry copper deposits, defined in terms of contained copper
metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene
to Pliocene. These giant deposits are clustered within three provinces, central Chile, northern Chile, and
southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina,
Irian Jaya, Mongolia, and Iran. Compressive tectonic environments, thickened continental crust, and active uplift
and erosion were associated with the formation of many of these deposits. Calc-alkalic magmas are most favorable
for the formation of giant porphyry copper deposits, although several of the largest systems are associated
with high K calc-alkalic intrusions.
The 25 largest gold-rich porphyry deposits are concentrated in the southwest Pacific and South America,
with other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits
formed in the last 13 m.y. The largest of the deposits are associated with high K calc-alkalic intrusions. Many
calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries.
In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges, seamount chains, and
oceanic plateaus beneath oceanic island and continental arcs. In several examples, these tectonic perturbations
have promoted flat-slab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with
the formation of well-endowed porphyry and/or epithermal mineral provinces. Similar tectonic features are inferred
to be associated with the giant porphyry copper-molybdenum provinces of northern Chile (Eocene-Oligocene) and southwest United States (Cretaceous-Paleocene).
Topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry
ore formation. Other factors, such as sutures in the overriding plate, permeability architecture of the upper
crust, efficient processes of ore transport and deposition, and, in some cases, formation and preservation of supergene
enrichment blankets are also vital for the development of high-grade giant ore deposits. A low-grade
geochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently, even in the most favorable geodynamic settings.
712 citations
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TL;DR: Long Valley caldera, a 17- by 32-km elliptical depression on the east front of the Sierra Nevada, was formed 0.7 m.y. ago during eruption of the Bishop tuff as mentioned in this paper.
Abstract: Long Valley caldera, a 17- by 32-km elliptical depression on the east front of the Sierra Nevada, was formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68-0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende-biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende-biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self-sealing of near-surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated northwest-trending Sierra Nevada frontal faults that tap hot water at depth.
556 citations
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TL;DR: The El Chichon trachyandesite was at a relatively low temperature in the range 750° to 850°C, with fO2 above the Ni-NiO buffer as mentioned in this paper.
429 citations
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TL;DR: In this paper, SHRIMP U-Pb zircon geochronology, combined with cathodoluminescence (CL) imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the Paleoproterozoic Jiao-Liao-Ji belt.
403 citations
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TL;DR: It is proposed that features of porphyritic andesite and dacite lavas that are rich in crystals and display a range of disequilibrium features can also be caused by convection within a magma body with a single composition, that is heated from below and cooled from above.
Abstract: Characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features, including reversely zoned crystals, resorption surfaces, wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix. These features are often interpreted as evidence of the mixing of magmas of contrasting composition, temperature and origin1,2. Here, however, we propose that such features can also be caused by convection within a magma body with a single composition, that is heated from below and cooled from above. We describe petrological observations of andesite lava erupted at the Soufriere Hills volcano, Montserrat, which indicate a heating event and the intermingling of crystals that have very different thermal histories. We present experimental data on a representative groundmass composition of this lava, which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature. The concept of convective self-mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions.
363 citations