Mineralogical constraints on the petrogenesis of trachytic inclusions, Carpenter Ridge Tuff, Central San Juan volcanic field, Colorado (original) (raw)
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Journal of Geophysical Research, 1990
Ma rhyolite lava and precursory non-welded tuff that form the Spor Mountain Formation in west-central Utah. The mafic inclusions are not lithic inclusions; no comparable volcanic unit was present at the surface when the Spor Mountain Formation erupted. Mineral and bulk compositions preclude liquid immiscibility. The mafic inclusions show clear morphologic and textural effects of magma mingling shortly before eruption of the rhyolite. Globular inclusions from both units are vesicular, phenocryst-poor, plagioclase-sanidine-clinopyroxeneorthopyroxene-magnetite-ilmenite latites and trachytes with quench temperatures of about 1000°C. Although overlapping in SiO 2 , TiO 2 , Zr, and Hf concentrations, inclusions from the underlying tuff lack negative Eu anomalies and are enriched in P 2 O 5 , K 2 O, A1 2 O 3 , Sr, Pb, Cr, and Ni whereas those hosted in the overlying lava have small negative Eu anomalies and two-fold enrichments in Fe 2 O 3 , MnO, HREE, Y, Ta, Th, Rb, and Cs. The most reasonable explanation for the differences between the two sets of inclusions lies in selective chemical exchange between the rhyolite lava and the mafic inclusions after eruption. Limited mechanical mixing occurred after the inclusions solidified and became chemically modified. The textures of the inclusions in the lavas and the elements selectively mobilized in the inclusions imply that vapor-phase transport occurred in this low-pressure volcanic environment. If such substantial variations in inclusion compositions can arise during what must have been a short period of time before chemical reactions were halted by rapid cooling, it seems unlikely that the compositions of mafic inclusions formed by magma mingling in slowly cooled granites preserve their original compositions, mineralogies, or information about their ultimate sources. Using the compositions of such chemically modified inclusions as end-members for mixing calculations may lead to erroneous results regarding the significance of magma mixing in plutonic rocks. CHRISTIANSEN AND VENCHIARUTTI LIMITS ON INFERENCES FROM INCLUSIONS 17,719 the Great Basin, the northern part of the Basin and Range province. The Cenozoic volcanic history of the Great Basin was reviewed by Best et ah [1989] and of the area around Spor Mountain by Lindsey [1982] and Shawe [1972]. Cenozoic magmatism in the northern Great Basin began about 42 Ma with the emplacement of a calc-alkaline sequence of intermediate-composition lavas, ash flows, and small intrusions. The oldest volcanic rocks near Spor Mountain consist of dacitic to andesitic lavas, agglomerates, and ash flow tuffs. The Drum Mountain Rhyodacite, a prominent member of this association, erupted to form a stratovolcano with peripheral lava flows. Lindsey [1982] reports a fission track age on zircon of 42 Ma for this unit. At 38 Ma (Joy Tuff) and again at 32 Ma (Dell Tuff) rhyolitic ash flows were erupted. Most of the tuffs are found only as relatively thin remnants of intracaldera deposits. After an 11 m. y. lull in magmatic activity in the region, the 21 Ma Spor Mountain Formation erupted on the western margin of the Caldera complex [Lindsey, 1982]. Scattered eruptions of rhyolite and basalt to basaltic andesite lavas occurred in this part of the eastern Great Basin after about 10 Ma, including the eruption of the Topaz Mountain Rhyolite in the adjacent Thomas Range and Keg Mountains. High-angle block faulting typical of Basin and Range extension formed between 21 and 7 Ma. The rhyolites of the Spor Mountain Formation take their name from Spor Mountain which is a block of tilted and intricately faulted lower and middle Paleozoic sedimentary rocks composed chiefly of carbonates (Figure 1). Numerous, relatively small plugs, dikes, and breccia pipes of Spor Mountain Formation rhyolite intruded the sedimentary sequence. Post eruption basin-and-range faulting has complicated the structure making it difficult to estimate the number of vents involved. Lindsey [1979] identified at least 11 vents, including breccia pipes. Eruptions of the Spor Mountain Formation commenced with emplacement of a series of ignimbrites, pyroclastic fall deposits, and pyroclastic surge deposits, and ended with extrusion of rhyolite lavas over the tuff [Bikun, 1980]. The tephra deposits and local accumulations of tuffaceous sandstone and conglomerate have been mapped as the beryllium tuff member of the Spor Mountain Formation [Lindsey, 1979]. The pyroclastic rocks overlie Paleozoic sedimentary rocks, the Drum Mountain Rhyodacite, and the fluvial sediments of the Spor Mountain Formation mentioned above. The upper part of the tuff was hydrothermally altered and mineralized (Be-U-F-Li-Mn) by fluids trapped beneath the impermeable lava cap [Lindsey, 1977; Burt and Sheridan, 1981]. Alteration mineral assemblages in the tuff include smectite resulting from incomplete argillization of glass, local sericite, and secondary potassium feldspar [Lindsey, 1977]. The tephra reaches a thickness of almost 100 m; a central welded zone is developed in thick sections [Williams, 1963] and basal zones at a few localities (D. A. Lindsey, written communication, 1989). The tuff contains lithic inclusions of sedimentary rocks entrained as the pyroclastic material moved through the vent. Especially in the upper 10 m of the tuff, carbonate lithic inclusions were altered to colorful nodules of silica minerals, fluorite, clay, manganese oxides, and bertrandite during mineralization of the tuff.
Journal of Volcanology and Geothermal Research, 2001
The Leyva Canyon Member of the Rawls Formation is a sequence of Oligocene (27.3±27.1 Ma) silicic lava, tuff, and volcaniclastic rock that comprise a trachytic shield volcano in the central Bofecillos Mountains of Big Bend Ranch State Park, Texas. This silicic unit developed within a volcanic ®eld that was otherwise dominated by silica-undersaturated, ma®c to intermediate lava. Quartz trachyte to low-silica rhyolite of the Leyva Canyon volcano appears to be the result of magma mixing between mantle-derived, alkalic ma®c magmas and peraluminous crustal melt (,40% crustal input), followed by ,65% fractional crystallization. The parental ma®c component was probably similar to silica-undersaturated, ma®c lavas of the Rawls Formation. Peraluminous, A-type high-silica rhyolite represents the earliest-erupted lavas of the Leyva Canyon volcano and is unrelated to quartz trachyte and low-silica rhyolite via fractional crystallization. The high-silica rhyolite provides evidence for an episode of crustal melting beneath the Leyva Canyon volcano.
Journal of Volcanology and Geothermal Research, 1978
. Magma mixing in mafic alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions. J. Volcanol. Geotherm. Res., 4: 315--331. Green clinopyroxenes, commonly rounded and anhedral and richer in Fe, Na and Mn than the pyroxenes of the surrounding groundmass are a common feature of mafic alkaline volcanic rocks (e.g. basanites, monchiquites, leucitites). Some are accompanied by one or more of the following phases: Fe-rich kaersutite and biotite, anorthoclase, sodic plagioclase, apatite, magnetite, sphene, which are believed to be cognate with the green pyroxenes. We review evidence that these minerals have crystallized from mugearite, trachyte or phonolite magmas, and their presence in mafic alkaline rocks is due to magma mixing. The intermediate and salic magmas may sometimes be generated at mantle depths, possibly by melting of mantle material enriched in Fe, Na and volatiles. *By evolved phases we mean those phases which have a Mg/(Mg+Fe) ratio less than 65 and are unlikely to have been in equilibrium with normal mantle material Walton and Arnold (1970) K. Hansen (personal communication) J. Gutzon Larsen , and personal communication) Brooks and Rucklidge (1973 C.IL Brooks, A.K. Pedersen and D.C. Rex (in preparation) Carmichael (1969), Smith and and found reverse-zoned clinopyroxenes in calc-alkaline volcanic rocks and Anderson and Wright (1971) in tholeiitic lavas from Hawaii, but these are not considered here as we restrict our discussion to alkaline rocks. However, these cases probably also reveal magma mixing.
A Complex Magma Mixing Origin for Rocks Erupted in 1915, Lassen Peak, California
Journal of Petrology, 1999
The eruption of Lassen Peak in May 1915 produced four volcanic INTRODUCTION rock types within 3 days, and in the following order: (1) hybrid Magma mixing is an important process among interblack dacite lava containing (2) undercooled andesitic inclusions, mediate magmas in volcanic arcs (e.g. Turner & (3) compositionally banded pumice with dark andesite and light Campbell, 1986; Philpotts, 1990, and references therein). dacite bands, and (4) unbanded light dacite. All types represent Laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption. They contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms (e.g. Cashman & Bergantz, 1991). However, use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite. The petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid-crystal mixtures at magmatic and partially hybridized with it. Phenocrysts from the dacite magma temperatures and pressures. If magmas mix incompletely, were reacted. Cooling, crystallization, and vesiculation of the hybrid i.e. mingle, features such as compositional banding or andesite magma converted it to a layer of mafic foam. The decreased undercooled inclusions are usually apparent, but if they density of the andesite magma destabilized and disrupted the foam. mix completely, mineralogical disequilibrium may be the Blobs of foam rose into and were further cooled by the overlying dacite magma, forming the andesitic inclusions. Disaggregation of only direct evidence for a mixing origin. andesitic inclusions in the host dacite produced the black dacite and If the thermal and compositional contrasts between light dacite magmas. Formation of foam was a dynamic process. two magmas are great and the ratio of silicic to mafic Removal of foam propagated the foam layer downward into the magma is large, there is little interaction between them, hybrid andesite magma. Eventually the thermal and compositional and the mafic magma is undercooled (e.g. Bacon, 1986). contrasts between the hybrid andesite and black dacite magmas were Many undercooled inclusions contain reacted phereduced. Then, they mixed directly, forming the dark andesite magma. nocrysts inherited from their host silicic magma (Heiken About 40-50% andesitic inclusions were disaggregated into the & Eichelberger, 1980), and Bacon (1986) pointed out host dacite to produce the hybrid black dacite. Thus, disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas. In general, the formation of undercooled an efficient magma-mixing process. Disaggregation of undercooled inclusions retards further mixing (Sakuyama, 1984; inclusions carrying reacted host-magma phenocrysts produces co-Thompson & Dungan, 1985; Sparks & Marshall, 1986; existing reacted and unreacted phenocryst populations. Koyaguchi & Blake, 1991). However, fragmentation and/ or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems
Introduction to Minerals, Inclusions and Volcanic Processes
Reviews in Mineralogy and Geochemistry, 2008
Minerals are intrinsically resistant to the processes that homogenize silicate liquids-their compositions thus yield an archive of volcanic and magmatic processes that are invisible at the whole rock scale. Minerals and their inclusions record diverse magma compositions, the depths and temperatures of magma storage, the nature of open system processes, and the rates at which magmas ascend. The potential for understanding volcanic systems through minerals and their inclusions has long been recognized (Sorby 1858). Sorby's (1863) study of James Hall's reversal experiments helped resolve the "basalt controversy" in favor of a volcanic origin, while zirkel's (1863) discovery of quartz within a volcanic rock helped tip the balance in favor of a magmatic origin for granite (Young 2003). Studies of phenocrysts have also long illustrated the importance of wall rock assimilation and magma mixing (e.g., Fenner 1926; Finch and Anderson 1930; Larson et al. 1938), and the potential for geothermometry (Barth 1934). Darwin's (1844) mineralogical field-studies in the Galapagos archipelago, followed by King's (1878) studies at Hawaii, also inaugurated the establishment of fractional crystallization as an important evolutionary process (Becker 1897; Bowen 1915). Recent advances in micro-analytical techniques open a new realm of detail, building upon a long history of mineralogical research; this volume summarizes some of this progress. Our summary focuses on volcanologic and magmatic processes, but the methods reviewed here extend well beyond terrestrial applications. Samples from the Stardust return mission, for example, show that olivine, plagioclase and pyroxene pervade the solar system (Brownlee et al. 2006)-while the topics covered here surely apply to all terrestrial-like planetary bodies, relevance may extend to a cosmic scale. Our more modest hope is that this volume will aid the study of disparate fields of terrestrial igneous systems, and perhaps provide a catalyst for new collaborations and integrated studies. OVerVIeW OF the VOLuMe Our review begins by tracing the origins of mineral grains, and methods to estimate pressures (P) and temperatures (T) of crystallization. Key to such attempts is an understanding of textures, and in her review, Hammer (2008) shows how "dynamic" experiments (conducted with varying P or T), yield important insights into crystal growth. Early dynamic experiments (e.g., Lofgren et al. 1974; Walker et al. 1978) have shown that porphyritic textures can result from a single episode of cooling. More recent experiments demonstrate that crystals can form during ascent due to loss of volatiles (Hammer and Rutherford 2002). Hammer (2008) describes these and other advances, and additional challenges that require new experimental The next four chapters document insights obtained from isotopic studies and diffusion profiles. Ramos and Tepley (2008) review developments of micro-analytical isotope measurements, which now have the potential to elucidate even the most cryptic of open system behaviors. 87 Sr/ 87 Sr ratios, for example, can be matched to dissolution surfaces to identify magma recharge events (Tepley et al. 2000). And 87 Sr/ 86 Sr-contrasts within and between phenocrysts allow differentiation between the roles of wall rock assimilation and enriched mantle sources to explain elevated 87 Sr/ 86 Sr (Ramos and Reid 2005). In the next chapter, Cooper and Reid
Journal of Petrology, 1997
The Cadillac Mountain intrusive complex is dominated by the INTRODUCTION Cadillac Mountain granite and a 2-3 km thick section of interlayered Enclaves occur in nearly all granites and have recently gabbroic, dioritic and granitic rocks which occurs near the base of been the focus of many studies [see reviews by Vernon the granite. The layered rocks record hundreds of injections of basaltic (1983) and Didier & Barbarin (1991)]. Although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma. Magmatic enclaves, ranging in different sources, there has been a growing consensus composition from 55 to 78 wt % SiO 2 , are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks. The most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments. Their compositions can be best explained by periodic (Vernon, 1984, 1990); these can appropriately be termed replenishment, mixing and fractional crystallization of basaltic magmatic enclaves. Their compositions are commonly magma at the base of the chamber. The intermediate to silicic intermediate but can vary from basaltic to highly silicic. enclaves formed by hybridization between the evolved basaltic magma Magmatic enclaves occur in all types of granite and are and resident silicic magma. There is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite. Instead, alkaline I-type granites. In spite of partial reequilibration, hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have Nd isotopic commagmas at the base of the chamber. Enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics, suggesting support their formation from mantle-derived magmas they were derived from a discrete batch of hybrid magma. Enclaves (Holden et al., 1987). were probably dispersed into a localized portion of the granitic The compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton, and they can only layer. rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma. There is ample petrographic and chemical evidence in many enclaves for hybridization (mixing of liquids and
Contributions To Mineralogy and Petrology, 1980
The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico. In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains ∼1% K2O at 60% SiO2. Trace element abundances of a typical ignimbrite with 73% SiO2 are Sr ∼ 225 ppm, Rb ∼130 ppm, Y ∼32 ppm, Th ∼12 ppm, Zr ∼200 ppm, and Nb ∼15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation. Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.
Journal of Geophysical Research, 1989
The Topopah Spring, Tiva Canyon, Rainier Mesa, and Ammonia Tanks tuffs are large-volume, silicic ash flow sheets that provide samples of four magmatic systems in southwestern Nevada. Successively erupted within a span of 2 m.y. from the same source area, they allow comparison of the sequential evolution of large, mature Cordilleran magmatic systems. Each large-volume sheet has a rhyolitic lower zone and quartz latitic upper zone. Coeval basaltic andesite and basalt show petrochemical continuity with these sheets and may represent mantle contributions that triggered eruptions of the midcrustal silicic portions. Abundances of phenocrysts and accessory phases increase upward with whole rock Fe (FeOt) from the base of all four sheets to maximum values unique for each system. Although maximum abundances of each mineral are unique for each sheet, each maximum occupies the same relative position within each sheet. High-temperature minerals such as plagioclase increase in abundance continuously with FeOt in each system, showing a decrease with FeOt only within basaltic andesite at the base of the Rainier Mesa system. Late crystallizing minerals such as quartz and sphene show maximum abundances at much lower FeOt, at or near the top of the rhyolitic zone.
Journal of Volcanology and Geothermal Research, 1994
Rhyolitic lavas and mafic inclusion-bearing dacites (MIBD) form the dominant products of the Monte Arci volcanic complex, one of the most active sites of volcanic activity in Sardinia during the Pliocene. The massif is composed of four distinct eruptive episodes (Phase 1: rhyolites; Phase 2: dacites and andesites; Phase 3: quartznormative trachytes; Phase 4: mafic lavas ranging from subalkaline to mildly alkaline). Monte Arci magmafism has been characterized by open-system behaviour, with both mantle and crustal contributions and magma mixing. Although the mafic products are restricted to the latest stage of activity, mafic inclusions are quite common in many rhyolites and dacites. The mineral assemblages of the inclusions are dominated by plagioclase + orthopyroxene + augite, with minor olivine, Fe-Ti oxides and variable amounts of residual trapped liquid giving rise to a fine-grained groundmass. They represent blobs of magma entrained in a partly molten state and provide evidence of a basaltic contribution to the petrogenesis of their enclosing lavas, both as parental magmas or as a source of heat for partial melting of crustal rocks.
The commingling of diverse magma types in the Flagstaff Lake Igneous Complex
1989
The Flagstaff Lake Igneous Complex is a member of an array of Acadian plutons which extend from the Katahdin batholith in the northeast to New Hampshire in the southwest. It is characterized by four major rock types: (1) gabbro, (2) granite, (3) garnet tonalite, and (4) trondhjemite. Field, petrographic, and geochemical evidence support the hypothesis that these diverse rocks existed as contemporaneous liquids, and that extensive mixing occurred at the time of emplacement. The dominant mafic mineral pairs in the gabbros are olivine-clinopyroxene, two pyroxenes, and pyroxenehornblende. The high Fe, low Ni and Cr contents of most of the gab bro sampled imply that it underwent substantial fractional crystallization prior to and during emplacement. The granitic rocks of the Flagstaff Lake Igneous Complex are heterogeneous, ranging from two-mica granites with equal amounts (-30%) of orthoclase, plagioclase, and quartz, to mafic diorites. These K-feldspar-bearing silicic rocks are exposed over-40% of the areal extent of the complex. Trondhjemite containing biotite, plagioclase, cordierite, and quartz, with minor muscovite and K-feldspar, is located along the margins of the intrusion, particularly in areas where the contact between the gabbro and granite intersects aluminous metapelitic wall rocks. The garnet tonalite is generally located in sections of the complex where the wall rocks are sulfidic, graphitic metasedimentary rocks. The tonalite is characterized by up to 60% almandine garnet, and oscillatory-zoned plagioclase, biotite, quartz, apatite, and zircon. The bodies of garnet tonalite are commonly, but not exclusively, located near the contacts between gabbro and country rock, and truncate the local strike of the country rocks.