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.

Giant Porphyry Deposits: Characteristics, Distribution, and Tectonic Controls

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.

Volcanism, structure, and geochronology of Long Valley Caldera, Mono County, California

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.

The 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: Mineralogy and petrology of the anhydritebearing pumices

Abstract: The 1982 eruptions of El Chichon Volcano produced three major pumice- and ash-fall layers. Fresh pumices from the three layers are indistinguishable porphyritic trachyandesites with 55.9 wt. % SiO2, 2.2% MgO, 2.8% K2O, 0.4% P2O5, and over 1.2% SO3. The pumices contain 58 wt. % crystals dominated by plagioclase (mode An43) and hornblende, with lesser amounts of augite, titanomagnetite, anhydrite, apatite, sphene, pyrrhotite, and biotite enclosed in a vesiculated matrix glass with 68 wt. % SiO2. Anhydrite forms subhedral to euhedral microphenocrysts without reaction coronas, as well as inclusions within the outer zones of major phenocrystic minerals. Inclusions of apatite and glass occur within anhydrite microphenocrysts. Anhydrite was crystallizing from the melt prior to eruption. Pumices resampled some ten months after the eruptions contain only 0.2 wt. % SO3, and have only scattered remnants of anhydrite and traces of gypsum lining vesicles. A single rainy season in Chiapas (>4 m yr−1 rainfall) was sufficient to remove most primary anhydrite, which may be a relatively common igneous mineral, although rarely preserved in the geologic record. El Chichon trachyandesites are relatively enriched in K2O, Rb, Sr, Th, U, Cs, and light rare elements compared to other andesites from Mexico and Central America. These enrichments may be related to the large distance from El Chichon to the Middle America Trench, or to subduction beneath Chiapas of the Tehuantepec Ridge, a major fracture zone of the Cocos Plate. Major- and trace-element analyses obtained by microprobe and instrumental neutron activation are given for all major minerals and glass, allowing trace-element partition coefficients to be calculated. Sphene and apatite contain high concentrations of rare earth elements, W, Th, and U. Sphene is also highly enriched in Hf and Ta. Prior to eruption, the El Chichon trachyandesite was at a relatively low temperature in the range 750° to 850°C, with fO2 above the Ni-NiO buffer. The melt was very water rich with some 4 to 10 wt. % H2O; the magmatic water content was 2–4 wt. %. The magma also contained an estimated 2.6 wt. % sulfur (as SO3). Of this amount, 1.2 wt. % resided in anhydrite crystals and SO2.

SHRIMP U–Pb zircon geochronology of the Liaoji granitoids: Constraints on the evolution of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton

Abstract: The Paleoproterozoic Jiao-Liao-Ji belt lies at the eastern margin of the Eastern Block of the North China Craton and is composed mainly of the Liaohe Group and the Liaoji granitoids. The Liaoji granitoids are divisible into pre-tectonic magnetite and hornblende/biotite monzogranitic gneisses and post-tectonic or anorogenic porphyritic monzogranites and alkaline syenites. 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. SHRIMP U–Pb zircon analyses reveal that the pre-tectonic magnetite monzogranitic gneisses were emplaced in the period 2176–2166 Ma and metamorphosed at 1914 Ma, and the pre-tectonic hornblende/biotite monzogranitic gneisses were emplaced at 2150–2143 Ma. These data demonstrate that the pre-tectonic monzogranitic gneisses of the Liaoji granitoids were not younger than the deposition of the Liaohe Group, as previously considered, since the latter contains a large amount of 2100–2000 Ma detrital zircons. In contrast, the pre-tectonic monzogranitic gneisses of the Liaoji granitoids or similar-aged granitoids may have been an important component of the provenance for the Liaohe Group. SHRIMP U–Pb zircon analyses also reveal that the post-tectonic porphyritic monzogranites and granites in the Jiao-Liao-Ji belt were emplaced at 1875 ± 10 and 1856 ± 31 Ma, respectively, simultaneously with the emplacement of the alkaline syenites that was dated at 1857 ± 20 and 1843 ± 23 Ma. Similar post-tecotnic or anorogenic granites are not only limited to the Jiao-Liao-Ji belt, but are also found in the North and South Korea, suggesting that the ∼1.85 Ga post-tectonic or anorogenic granitic magmatic event may have occurred in both the Korean Peninsula and the Eastern Block of the North China Craton. The SHRIMP zircon ages of this study combined with lithological, structural, metamorphic and geochemical data is consistent with a rift closure model which suggests that the Archean complexes on the northern and southern sides of the Jiao-Liao-Ji belt were originally situated on a single continental block that underwent Paleoproterozoic rifting, associated with the formation of the Liaohe Group and Liaoji granitoids, and closed upon itself at ∼1.9 Ga.

Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers.

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.