Silicic Magma Chambers Research Articles

Tectonic controls on magmatism in The Geysers–Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

Isotopic and chemical evolution of the post-caldera rhyolitic system at Long Valley, California

Post-caldera rhyolites of the Long Valley magmatic system are chemically less evolved than pre-caldera rhyolites or the initial phases of the Bishop Tuff and record temporal variations in composition which imply open-system magma differentiation involving magma replenishment. All post-caldera rhyolites lie on a well-defined Pb–Sr isotope mixing line between the ∼700 ka Resurgent Dome and the recent Inyo lava domes, precluding a simple cogenetic origin. Coherent temporal trends in Pb and Sr isotopes provide compelling evidence for the near-continuous addition of magma into a silicic magma chamber that was residual after Bishop Tuff eruption. Nd isotope ratios do not record such consistent variations, arguing against their use as a proxy for major additions of new magma and hence as a monitor of potential volcanic hazard. The lowering of 87Sr/86Sr with time demonstrates that there was little crustal interaction and that the Nd isotope composition of the added magmas was variable. The rhyolites of Mammoth Knolls are the most differentiated and define an Rb–Sr isochron of 277±124 ka. These data are consistent with stratification of the magma chamber and subsequent isolation of the upper, more evolved, sections at ∼275 ka. Post 400 ka, rhyolites become chemically more varied, supporting evidence of stratification. The high- and low-silica rhyolites record distinct temporal Sr isotope evolution, implying that the low-silica rhyolites sampled the bulk of the magma chamber whereas the high-silica rhyolites represent the upper isolated, parts of the magma chamber, where they resided for >30 and >300 kyr. After the eruption of the Bishop Tuff, the Long Valley magma chamber was well mixed for the first ∼400 kyr of its evolution and produced rhyolitic magmas at an average of ∼0.0001 km3/year. Upon stratification post 400 ka, tens of km3 of chemically more evolved magma were rapidly produced. Magma addition to the system was at a constant rate and more frequent than eruptions.

Felsic Enclave Swarms in the Gouldsboro Granite, Coastal Maine: A Record of Eruption Through the Roof of A Silicic Magma Chamber

Prominent zones of large globular, fine‐grained, felsic enclaves occur at the highest exposed levels of the Gouldsboro granite along the coast of Maine. The physical characteristics of these enclaves and their chemical relations to the host granite indicate that they represent globules of resident magma that were trapped, probably during eruption, in the crystal mush at the top of the magma chamber. These zones can appropriately be termed “eruption trails.” Enclave compositions provide a record of magma compositions within the chamber at the time of eruption. They fall into two groups (high‐K and low‐K with similar wt % SiO2), suggesting that magma in the chamber consisted of discrete compositional batches. The distribution of these two types of enclaves within the swarms is consistent with the low‐K source magma residing as a layer beneath the high‐K source magma. Chilled and vesiculated inclusions of basalt occur only in the low‐K enclaves. Their presence there suggests that basaltic magma was injected into the base of the chamber just prior to and may have triggered the eruption. The low‐K silicic magma probably developed by selective exchange of alkalies between normal high‐K silicic magma and periodic injections of basaltic magma. Evidence for these injections is seen in the chilled gabbroic sheets at the base of the Gouldsboro granite. Similar large felsic enclaves are known in other shallow‐level granites; many of these may represent centrally derived magma trapped during transport out of the chamber in an eruption rather than, as they have commonly been interpreted, portions of chilled margins engulfed and remobilized during granite emplacement.

El Salvador, Chile Porphyry Copper Deposit Revisited: Geologic and Geochronologic Framework

The Eocene (42 to 41 Ma) El Salvador porphyry copper deposit in the Indio Muerto district, northern Chile (26° 15′ S Lat.), formerly thought to have formed at the culmination of a 9-m.y. period of episodic magmatism, is shown by new mapping, U-Pb and K-Ar geochronology, and petrologic data to have formed during the younger of two distinct but superposed magmatic events-a Paleocene (∼63 to 58 Ma) and an Eocene (44 to 41 Ma) event. In the district, high-K Paleocene volcano-plutonic activity was characterized by a variety of eruptive styles and magmatic compositions, including a collapse caldera associated with explosive rhyolitic magmatism (El Salvador trapdoor caldera), a post-collapse rhyolite dome field (Cerro Indio Muerto), and andesitic-trachyandesitic stratovolcanos (Kilometro Catorce-Los Amarillos sequence). Pre-caldera basement faults were reactivated during Paleocene volcanism as part of the collapse margin of the caldera. Beneath Cerro Indio Muerto, where the porphyry Cu deposit subsequently formed, the intersection of two major basement faults and the NNE-striking rotational axis of tilted ignimbrites of the Paleocene El Salvador caldera localized emplacement of post-collapse rhyolite domes and peripheral dikes and sills. Subsequent Eocene rhyolitic and granodioritic-dacitic porphyries intruded ~14 m.y. after cessation of Paleocene magmatism along the same NNE-striking structural belt through Cerro Indio Muerto as did the post-collapse Paleocene rhyolite domes. Eocene plutonism over a 3-m.y. period was contemporaneous with NW-SE-directed shortening associated with regional sinistral transpression along the Sierra Castillo fault, lying ∼10 km to the east. Older Eocene rhyolitic porphyries in the Indio Muerto district were emplaced between 44 and 43 Ma, and have a small uneconomic Cu center associated with a porphyry at Old Camp. The oldest granodioritic-dacitic porphyries also were emplaced at ∼44 to 43 Ma, but their petrogenetic relation to the rhyolitic porphyries and younger granodioritic-dacitic porphyries in the district is unclear. The main porphyry Cu-Mo-related granodioritic-dacitic stocks in Quebrada Turquesa on Cerro Indio Muerto intruded, cooled, and were mineralized within ∼1 m.y. between 42 and 41 Ma. Volumetrically minor late- to post-mineral porphyries are slightly more mafic than earlier granodioritic-dacitic porphyries, a compositional trend possibly repeated on several scales and more than once over the 3-million-year Eocene magmatic history of the Indio Muerto district. This compositional trend requires either addition of basaltic material into an open-system silicic magma chamber or tapping of progressively deeper levels of a vertically zoned magma chamber. Eocene porphyry magmas were more hydrous and their residual source mineralogy richer in garnet than the relatively anhydrous Paleocene rocks, whose source was rich in pyroxene. The presence of inherited zircons in Paleocene and Eocene rocks requires interaction with crustal rocks of Paleozoic and/or Proterozoic age. Paleocene and Eocene igneous rocks in the Indio Muerto district were emplaced during distinct magmatic-tectonic events that are unrelated, although spatially associated. The districtscale Paleocene and Eocene eruptive styles and geochemical and mineralogic characteristics mimic characteristics of similar-aged igneous rocks throughout northern Chile (20°30′ S Lat. to 27° S Lat.), attesting to the regional nature of the Paleocene and Eocene events. Porphyry Cu mineralization in the district furthermore is associated not only with an Eocene granodioriticdacitic (42 to 41 Ma) complex, but also with one of an older Eocene (44 to 43 Ma) rhyolitic porphyry, implying that a long period of precursor magmatism is not required for generation of the El Salvador porphyry Cu-Mo deposit. Rather, the episodic magmatism preceding porphyry Cu mineralization reflects repeated structural localization through time of superimposed highlevel volcano-plutonic complexes in an active magmatic arc.

Hornblende gabbro sill complex at Onion Valley, California, and a mixing origin for the Sierra Nevada batholith

The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite, individually as thick as 1.5 m, form the upper 200 to 300 m of the complex. Thicker, multiply-injected sills, as well as mafic stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad, lie near the base of the sheeted sill suite. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against septa, millimeters to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling, show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying more silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic magmas appear to have been produced from an enriched mantle source with 87Sr/86Sr ∼0.7065, ɛNd ∼−4.3, 206Pb/204Pb ∼18.6, 207Pb/204Pb ∼15.6, 208Pb/204Pb ∼38.6. Although crystal fractionation contributed to forming the sill suite and the associated cumulates, nearly constant concentrations of Na2O, P2O5, Nb, Zr, and light rare earth elements in the sills indicate that mixing between sill basaltic and more evolved septa magmas was important for producing sills with andesitic compositions. Average Sierran granodiorite major and trace element concentrations are readily reproduced by a simple mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite. This result supports the inference that Sierran granitoids formed chiefly by mixing between crustal and mantle-derived magmas, although in some cases these crustal melts may have been derived by refusion of earlier mafic intrusions near the base of the crust. The common mafic inclusions (enclaves) in Sierran granodiorites bear a superficial resemblance to Onion Valley mafic sills; however, high concentrations of lithophile elements in the inclusions point to extensive chemical exchange between inclusions and their host magmas. The prevalence of hornblende-rich mafic intrusive rocks at Onion Valley, elsewhere in the Sierra Nevada, and in other shallow subduction batholiths stems from two effects of high melt water concentrations (∼4–6 wt% H2O). The hydrous parent basaltic and basaltic andesite magmas had low liquidus temperatures, compared to nearly dry basaltic melts, and thus were chilled less during ascent through the crust and were more capable of ascent as liquids. More importantly, their high water concentrations led to low melt densities, higher than granitoid liquids, but comparable to or less dense than partly solidified granitoid magmas. Thus, the hydrous basaltic and basaltic andesite magmas were neutrally or positively buoyant and were capable of penetrating and rising through partly crystallized granitoids and their partly molten source regions to reach upper crustal emplacement levels. Drier basaltic magmas were probably abundant at depth and contributed heat and mass to granite generation, but were insufficiently buoyant to ascend to shallow levels.

Magma mixing by convective entrainment

RECENT studies of volcanic eruptions have brought to light a puzzling sequence: the ejection of dense and mixed liquids followed by a larger volume of unmixed, less dense liquid1–4. Although these studies have shown that the mixing is associated with the injection of basaltic liquid into a more silicic magma chamber, a paradox remains. Basaltic magma is denser than silicic magma, and should flow along the floor of the magma chamber. This configuration impedes mixing of the two liquids, and appears incompatible with the early eruption of basaltic liquid from the top of the chamber. Previously proposed mixing processes explain neither the eruption sequence nor the eruption of dense liquid during the time of lowest eruption rate1–8. We report here fluid-mechanical experiments that suggest a solution: a thermal plume forms over the replenishment inlet, dragging basaltic liquid upward by viscous coupling, and producing mixing only in a localized area without affecting the bulk of the chamber.

Mafic-silicic layered intrusions: the role of basaltic injections on magmatic processes and the evolution of silicic magma chambers

ABSTRACT:Plutonic complexes with interlayered mafic and silicic rocks commonly contain layers (1–50 m thick) with a chilled gabbroic base that grades upwards to dioritic or silicic cumulates. Each chilled base records the infusion of new basaltic magma into the chamber. Some layers preserve a record of double-diffusive convection with hotter, denser mafic magma beneath silicic magma. Processes of hybridisation include mechanical mixing of crystals and selective exchange of H2O, alkalis and isotopes. These effects are convected away from the boundary into the interiors of both magmas. Fractional crystallisation aad replenishment of the mafic magma can also generate intermediate magma layers highly enriched in incompatible elements.Basaltic infusions into silicic magma chambers can significantly affect the thermal and chemical character of resident granitic magmas in shallow level chambers. In one Maine pluton, they converted resident I-type granitic magma into A-type granite and, in another, they produced a low-K (trondhjemitic) magma layer beneath normal granitic magma. If comparable interactions occur at deeper crustal levels, selective thermal, chemical and isotopic exchange should probably be even more effective. Because the mafic magmas crystallise first and relatively rapidly, silicic magmas that rise away from deep composite chambers may show little direct evidence (e.g. enclaves) of their prior involvement with mafic magma.

Chemical and hydrogen isotope evidence for in situ dehydrogenation of biotite in silicic magma chambers

To examine the potential for volatile fluxing of magma chambers by in situ degassing of hydrous minerals, we obtained complete chemical analyses for biotite separates from silicic lavas. The separates exhibit unusually low H 2 O contents that inversely correlate with host lava temperatures, high Fe 3+ /Fe 2+ ratios that inversely correlate with host lava oxygen fugacities, and the highest δ D values yet reported for biotite from any silicic igneous rock (up to −19‰). These results are direct evidence for selective loss of protium ( 1 H) from biotite during dehydrogenation in magma chambers heated from below by intrusion of mafic magma. The maximum P Δ V energy generated from dehydrogenation alone can approach 2 × 10 3 joules per kilogram of magma. This finding provides support for the concept that injection of mafic magma coupled with sudden degassing of hydrous minerals in a volatile-rich magma chamber can increase pressure, and thus enhance the possibility of initiating a volcanic eruption.

Late Quaternary rhyolitic eruptions from the Acigöl Complex, central Turkey

Two major rhyolitic tephra units (Lower and Upper Acigöl Tuffs) have been erupted from the Acigöl Complex. The Lower Acigöl Tuff (>13km 3 uncompacted volume) was erupted more than c. 180 ka ago, forming an extensive Plinian pumice-fall deposit with associated ashfall and ignimbrite. Well-developed ignimbrite veneer facies are preserved where the pyroclastic flows passed over hilly topography. Following the eruption, a thick rhyolite coulée (Bogazköy obsidian) was extruded. The Upper Acigöl Tuff eruption (age 70 < t< 150 ka) generated a second major Plinian deposit, an ignimbrite preserved up to 24 km from the vent, and an 80 m thick proximal succession of surge and fallout deposits; it resulted in collapse of the 6×5 km Acigöl caldera. The two major tuffs are chemically similar but can be distinguished on the basis of lithic populations. Subsequent rhyolitic activity formed the c. 0.25 km 3 dome of Kocadag Tepe (c.70ka) and five dome/maar clusters (c. 20 ka). Late Quaternary and/or Holocene basalts and andesites surround the Acigöl caldera, which developed in a zone of sinistral shear associated with approximate N-S compression. Vent distribution patterns, the eruption of young basalts, and the crystal-poor nature of the c. 20 ka rhyolites are consistent with basaltic stoking of a sizeable silicic magma chamber beneath the Acigöl Complex.

Genesis of Alkaline and Peralkaline Syenite-Granite Series: The Kharitonovo Pluton (Transbaikalia, Russia)

The Kharitonovo pluton is located in the central part of the large Mongolian-Transbaikalian province of Late Paleozoic alkaline syenites and granites. The province stretches for a distance of almost 2000 km and contains over 350 plutons. The Kharitonovo pluton occupies an area of $$230 km.^{2}$$ It is made up of A-type granitoids forming two successive alkaline and peralkaline syenite-granite series; syenites largely predominate over granites. Rocks of both series are characterized by abundant mesoperthitic feldspar (about 90 vol % in syenites, more than 60% in granites) while plagioclase is almost absent. In the peralkaline series mafic minerals are riebeckite-arfvedsonite, kataphorite and aegirine with secondary annitic biotite. In the alkaline series edenite and iron-rich biotite are the major mafic minerals. In the alkaline series coeval mafic rocks commonly occur in the form of synplutonic composite dikes; they indicate at least two stages of mafic magma injection into the silicic magma chamber. Major and trace element data are used to test various petrogenetic models for the origin of the syenites and granites. Of the alternatives considered, it is most likely that each series resulted from crystal fractionation of syenite magma. Mass balance calculations suggest that in the early alkaline series, crystal fractionation was probably combined with mixing of felsic and mafic magmas, a conclusion also supported by field evidence. The peralkaline series syenite parental magma could have been produced by partial melting (about 20%) of the earlier alkaline syenites. In this respect alkaline syenites can be regarded as parental rocks for the whole pluton. We suggest that the alkaline syenite magma originated via two possible petrogenetic schemes: (1) partial melting of deeply buried crustal rocks or (2) crystal fractionation of hybrid melt produced by mixing of subalkaline basaltic magma (80%) with about 20% of silicic lower crustal melt. In either case some additional input of potassium and possibly other incompatible elements is required in order to achieve the observed composition.

40Ar/39Ar laser probe ages of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States

Laser probe Ar-40/Ar-39 dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 +/- 1 to 761 +/- 1 ha, Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 +/- 0.03 to 2.3 +/- 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts similar to 1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

A model of reverse differentiation at Dikii Greben' Volcano, Kamchatka: progressive basic magma vesiculation in a silicic magma chamber

Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.

Silicic Magma Chambers as Traps for Basaltic Magmas: The Cadillac Mountain Intrusive Complex, Mount Desert Island, Maine

The Cadillac Mountain intrusive complex, located on Mount Desert Island, Maine, provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma. The complex consists of three units: the Cadillac Mountain granite (CMG), the Somesville granite (SG), and gabbro-diorite (G-D). Basaltic magmas were emplaced before, during, and after the emplacement and crystallization of the granitic plutons, and the chambers of silicic magma acted as traps for basaltic magma. Basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro, while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma. The G-D unit (about 1.5 km thick) consists of interlayered gabbroic, dioritic, and granitic rocks. Although thin (<1 m thick) gabbroic layers in the G-D unit are typically chilled on all margins, thicker layers of gabbro commonly form the lower parts of macrorhythmic units that may grade upward from basally chilled gabbro through hybrid dioritic rocks to granite. These macrorhythmic units provide a cumulate record of the temporary stratification of mafic and silicic magmas in the CMG chamber. Widespread, small mafic enclaves occur at all levels of the CMG and were probably generated along turbulently stirred double-diffusive boundaries between mafic and silicic magma. Heat from basaltic infusions promoted convection in the overlying silicic magma, and this convection distributed enclaves thoughout the CMG chamber. Scarce, finegrained enclaves of hybrid, intermediate rock in the CMG may represent disrupted portions of a hybrid magma that evolved near the base of the chamber due to repeated infusions of basaltic magma. Some zones of transgressive granophyre in the CMG probably represent highly silicic liquids that evolved within the CMG magma chamber. The Somesville granite may represent a mixture of magma that evolved within the CMG chamber and injections of new silicic magma from below.

40Ar/39Ar chronology and volcanology of silicic volcanism in the Davis Mountains, Trans-Pecos Texas

Field studies and 40 Ar/ 39 Ar dating reveal that silicic volcanism in the Davis Mountains part of the Trans-Pecos Texas volcanic field occurred in six episodes at 0.3 m.y. intervals between 36.8 and 35.3 Ma. Additionally, two groups of silicic intrusions were emplaced at 34.6 and 32.8 Ma. This episodicity is similar to that determined for volcanic fields dominated by ash-flow tuffs, yet voluminous, extensive silicic lavas are considerably more abundant than tuffs in the Davis Mountains, by number of flows and by volume. The preponderance of voluminous silicic lavas over tuffs most likely reflects low water contents and high temperatures of the alkalic and commonly peralkaline Davis Mountains magmas. The earliest episode, at 36.8 Ma, included a widespread and voluminous (possibly >1,000 km 3 ) suite of rhyolite and quartz trachyte lavas, several rhyolite domes, and a strongly rheomorphic, peralkaline ash-flow tuff erupted from a caldera in the northern Davis Mountains. The lava suite extends well beyond the Davis Mountains. Silicic lavas of all episodes probably erupted from widespread, fissure vents. The 36.5 Ma episode consisted of rhyolite to quartz trachyte lavas, also extensive and voluminous (∼200 km 3 ). The 36.3 Ma episode consisted of rhyolite to trachyte tuffs and lavas erupted from a central vent volcano in the southern Davis Mountains. The 35.9 Ma episode consisted of a single, moderately large (∼50 km 3 ) rhyolite lava and a small-volume ash-flow tuff erupted from a caldera in the western Davis Mountains. Rocks emplaced during the 35.6 Ma episode were also rhyolites, including an enigmatic rock that may be strongly rheomorphic ash-flow tuff or a combination of tuff and lava, followed by definite lavas. The 35.3 Ma episode consisted of two ash-flow tuffs, one of which is strongly rheomorphic, and additional voluminous rhyolite lavas (∼120 km 3 ). The rheomorphic tuff erupted from a caldera in the southwestern Davis Mountains. The source of the other tuff is probably in the western Davis Mountains. Intermediate and mafic rocks are minor, except around the southeastern flank of the Davis Mountains, where basalt is abundant. Mafic lavas erupted only during gaps in the silicic activity and on the flanks of the Davis Mountains. Nevertheless, basaltic magma probably drove the silicic magmatism, either by differentiation or by crustal melting, and was present throughout the time of Davis Mountains activity but could not penetrate the low-density silicic magma chambers until they cooled and solidified. The time required for cooling and solidification appears to be 0.1-0.2 m.y.

The Pleasant Bay Layered Gabbro--Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber

The Pleasant Bay Layered Gabbro--Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber

Replenishment and crystallization in epicontinental silicic magma chambers: evidence from the Bandelier magmatic system

The Cerro Toledo Rhyolite (Jemez volcanic field, New Mexico) records the volcanism that occurred after the eruption of the Lower Bandelier Tuff ignimbrite, which formed Toledo caldera at 1.51 ± 0.03 Ma. Pumice samples from the top of this ignimbrite and from the base of the Cerro Toledo Rhyolite tephra sequence are stratigraphically and geochemically distinct from younger Cerro Toledo Rhyolite tephras that overlie this basal suite. All samples exhibit decreasing La/Lu with increasing Lu concentrations. For a given Lu content, however, the Lower Bandelier Tuff and basal member Cerro Toledo Rhyolites have higher La/Lu compared to overlying Cerro Toledo Rhyolites. This difference can be explained either by (1) differentiation of a single batch of magma in which trace mineral solubilities increased with time due to higher volatile contents, or (2) replenishment of the magma chamber by an input of silicic magma after caldera collapse at 1.51 Ma. Large-scale replenishment of epicontinental silicic magma chambers may be restricted to timescales of 10 5 years.

Basaltic injections into floored silicic magma chambers

Recent studies have provided compelling evidence that many large accumulations of silicic volcanic rocks erupted from long‐lasting, floored chambers of silicic magma that were repeatedly injected by basaltic magma. These basaltic infusions are commonly thought to play an important role in the evolution of the silicic systems: they have been proposed as a cause for explosive silicic eruptions [Sparks and Sigurdsson, 1977], compositional variation in ash‐flow sheets [Smith, 1979], mafic magmatic inclusions in silicic volcanic rocks [Bacon, 1986], and mixing of mafic and silicic magmas [Anderson, 1976; Eichelberger, 1978]. If, as seems likely, floored silicic magma chambers have frequently been invaded by basalt, then plutonic bodies should provide records of these events. Although plutonic evidence for mixing and commingling of mafic and silicic magmas has been recognized for many years, it has been established only recently that some intrusive complex originated through multiple basaltic injections into floored chambers of silicic magma [e.g., Wiebe, 1974; Michael, 1991; Chapman and Rhodes, 1992].

Evidence for ascent of differentiated liquids in a silicic magma chamber found in a granitic pluton

ABSTRACTFluid dynamic modelling of crystallising calc-alkalic magma bodies has predicted that differentiated liquids will ascend as boundary layers and that accumulation of these buoyant liquids near chamber roofs will result in compositionally stratified magma chambers. This paper reports physical features in La Gloria Pluton that can be interpreted as trapped ascending differentiated liquids. Leucogranitic layers decimetres thick, which are locally stratified, are trapped beneath overhanging wall contacts. The same felsic magmas were also preserved where they were injected into the wall rocks as dykes and as large sill complexes. These rocks do not represent differentiated magmas produced by crystallisation along the exposed walls because the felsic layers occur at the wall rock contact, not inboard of it. Rather, we speculate that evolved felsic liquids are generated by crystallisation all across the deep levels of chambers and that initial melt segregation occurs by flowage of melt into tension fractures. Melt bodies so formed may be large enough to have significant ascent velocities as diapirs and/or dykes. The other way in which the leucogranite occurrence is at variance with the convective fractionation model is that the ascending liquids did not feed a highly differentiated cap to the chamber, as the composition at the roof, although the most felsic in this vertically and concentrically zoned pluton, is considerably more mafic than the trapped leucogranitic liquids. This suggests that these evolved liquids were usually mixed back into the main body of the chamber. Backmixing may be general in continental-margin calc-alkalic magmatic systems, which, in contrast to those in intracontinental settings, rarely produce volcanic rocks more silicic than rhyodacite. That the highly differentiated liquids are preserved at all at La Gloria is a result of the unusual stepped nature of the contact and the entirely passive mode of emplacement of the pluton, which, in contrast to ballooning in place, does not result in wall zones being “scoured”.

Multistage crystallization of tonalitic enclaves in granitoid rocks (Hercynian belt, Spain): implications for magma mixing

The crystallization sequence, nucleation density and crystal index (n = nucleation density/mode) of tonalitic enclaves from different granitoids of the Hercynian orogeny indicate that they are pieces of magma partially crystallized in the interior of synplutonic bodies of basic magma, which was injected into a silicic magma chamber. A rapid cooling stage can be identified from the high nucleation density and high n-values of minerals. A final stage of slow cooling, is identified by a low nucleation density and low n-values. At this stage a residual melt crystallized in thermal equilibrium to the cooling rate of the whole magma chamber. Thermal equilibrium and convection are necessary conditions for mixing. Enclaves can be interpreted as the remaining, non-disaggregated portions of an early injected mafic magma, which in turn underwent hybridization during injection. Their presence indicates the existence of magma mixing processes and the possibility that the host granitoid was a hybrid rock.

The origin and implications of undercooled andesitic inclusions in rhyolites, clear lake volcanics, California

At Clear Lake, California, undercooled andesitic inclusions are common in rhyolitic to dacitic lava flows and pyroclastic deposits. One group of rhyolite lava flows contains about 1 vol % undercooled andesitic inclusions &lt;2 cm in size. The inclusions are evenly distributed in several flows and do not vary significantly in size or abundance. They consist of plagioclase, orthopyroxene, ilmenite, apatite needles, and vesicular rhyolitic glass ±olivine±clinopyroxene. Microphenocryst size in these small inclusions correlates positively with inclusion size, suggesting that inclusions have not undergone extensive disaggregation since formation. Texturally similar inclusions up to 50 cm in size occur in associated pyroclastic deposits. None of the inclusions represent the mafic end‐member of the system. Rather, they represent intermediate magmas formed by liquid‐crystal fractionation and hybridization of distinct andesitic magmas at or near the base of a silicic magma chamber. The large inclusions are transitional in composition and mineralogy between Clear Lake basaltic andesites and smaller inclusions, which show more evidence of hybridization. These differences suggest similar but distinct sources and mechanisms of formation for the large and small inclusions. The small inclusions probably formed by episodic mingling across an interface or hybrid zone separating rhyolitic and andesitic magmas. We speculate that earthquakes generated along the San Andreas fault system may have triggered mingling events. This occurrence demonstrates that dispersion of undercooled intermediate to mafic inclusions in silicic magma chambers is the dominant mode of mafic‐felsic magma interaction in some systems.