Robert A. Wiebe

Experimental study of liquid evolution in an Fe-rich, layered mafic intrusion: constraints of Fe-Ti oxide precipitation on the T-fO2 and T-ϱ paths of tholeiitic magmas

The Newark Island layered intrusion, a composite intrusion displaying a similar fractionation sequence to the Skaergaard, has both dikes which preserved liquids fed into the intrusion and chilled pillows of liquids resident in the chamber. This study reports experimentally determined one atmosphere liquid lines of descent of these compositions as a function of oxygen fugacity which varies from QFM (quartz-fayalite-magnetite) to 0.5 log10 units above IW (iron-wustite). These experiments reveal a strong oxygen fugacity dependence on the order of appearance and relative abundances of the Fe−Ti oxide minerals. Titanomagnetite saturates prior to ilmenite at QFM, but the order is reversed at lower oxygen fugacities. In the layered series of the Newark Island intrusion, ilmenite arrives shortly before titanomagnetite and the titanomagnetite/ilmenite ratio decreases monotonically after the cumulus appearance of titanomagnetite. Comparison of the crystallization sequence in the intrusion with that of the experiments requires that the oxygen fugacity in the intrusion increased relative to QFM before titanomagnetite saturation and decreased afterward, but always remained between the QFM and IW buffers. Similar trends in the modes of the Fe−Ti oxides (ilmenite and titanomagnetite) in the Skaergaard, Kiglapait, and Somerset Dam intrusions along with Fe2O3/FeO ratios in MORBs suggest that such a temperature-oxygen fugacity path may be typical of tholeiitic magma differentiation. Calculations of the temperature-density paths of the experimental liquids indicate that, at all possible oxygen fugacities, the density must have decreased abruptly after Fe−Ti oxide saturation. Accordingly, liquids replenishing the intrusion after Fe−Ti oxide saturation should pond at the bottom of the chamber, quenching against older cumulates. Field observation at the Newark Island intrusion confirm this prediction. The similarities in the fractionation paths of several other layered intrusions to that of the Newark Island intrusion suggest that the density of the liquids in these intrusions also decreased after Fe−Ti oxide saturation. Experiments on a suggested initial Skaergaard liquid are consistent with this model.

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

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 H 2 O, 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.

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.

Mafic injections, in situ hybridization, and crystal accumulation in the Pyramid Peak granite, California

Located in the northern Sierra Nevada, the Middle Jurassic Pyramid Peak granite and associated dioritic rocks display superbly exposed interlayered sheets of mafic, felsic, and hybrid rocks. The mafic-hybrid layers show a consistent asymmetry (sharply chilled on one side and gradational through hybrids to granite on the other) and dip to the southwest. Load casts and silicic pipes along the chilled, mafic northeastern margins of the sheets indicate that depositional way-up is to the southwest and that the adjacent Mount Tallac “roof pendant” was the original floor of the intrusion. Orientations of the pipes indicate that the layers were tilted after deposition and solidification. We interpret the sheets to be intramagmatic flows that formed by injections of mafic magma into a silicic magma chamber followed by flow across the crystal-rich base of that chamber. The granitic and hybrid rocks interlayered with the mafic sheets represent material that accumulated on the chamber floor between episodes of mafic replenishment. Most contamination of the mafic sheets probably occurred because of flow-front instabilities generated as the mafic magma entered and spread across the chamber floor. The upward gradation from diorite to granite with decreasing abundance and size of mafic enclaves can best be explained by convective stirring at the upper boundaries of the dioritic replenishments. The Pyramid Peak granite was probably initiated as a thin, concordant sheet intruding into gently dipping Jurassic supracrustal rocks; the pluton was constructed incrementally with many replenishments of felsic and mafic magma and dominantly solidified by accumulation of crystals on the floor of the chamber. The present steep dips of the layers suggest that downward warping of the chamber floor may have been a factor in accommodating the incrementally constructed pluton.

Magma Mingling in Dikes and Sills

Dikes in which two different liquids flowed simultaneously, usually called composite dikes, naturally fall into two classes depending on which lithology forms the contact with the country rock, and hence, which liquid was the first to enter the fracture. In these two kinds of dikes, the structures formed by the mingling of the two liquids differ. Dikes in which the more basic liquid entered first have contacts between the two lithologies that are nearly planar, and parallel to the dike walls, whereas the more basic lithology forms discrete pillows in those dikes in which the more silicic liquid entered first. Experiments indicate that these pillows probably form from a flow-front instability that develops when a liquid invades another of higher viscosity between two parallel rigid walls. We provide scalings for the critical flow rate for the onset of this instability, the time required for the instability to develop, and the wavelength that is selected. These scalings are consistent with field observations.

Slow, dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion, Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.

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].

Granites, dynamic magma chamber processes and pluton construction: the Aztec Wash pluton, Eldorado Mountains, Nevada, USA

The mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber. Tilting has exposed a 5-km section through the GZ and adjacent portions of the HZ. A porphyry is interpreted as a remnant of a chilled roof zone that marks the first stage of felsic GZ intrusion. Subsequent recharging by felsic and mafic magma, reflected by repeated cycles of crystal accumulation and melt segregation in the GZ and emplacement of mafic flows in the HZ, rejuvenated and maintained the chamber. Kilometre-scale lobes of mafic HZ material were deposited as prograding tongues into the GZ during periods of increased mafic input. Thus, they are lateral equivalents of the cumulate GZ granites with which they interfinger. Conglomerate-like units comprising rounded, matrix-supported intermediate clasts in cumulate granite are located immediately above the lobes. These ‘conglomerates’ appear to represent debris flows shed from sloping upper surfaces of the lobes. Thus, the GZ can be viewed as comprising distal facies, remote from the site of mafic recharging in the HZ, and the HZ as comprising proximal facies. Elemental chemistry suggests that the GZ cumulate granites represent a second-stage accumulation from an already evolved melt, and that coarse, more mafic, feldspar+biotite+accessory mineral ± hornblende rocks trapped between mafic sheets in the HZ are the initial cumulates. Fractionated melt accumulated roofward and laterally, and was the direct parent of the ‘evolved’ GZ cumulates. The most highly fractionated, fluid-rich melts accumulated at the roof.

Discrete stoping events in granite plutons: A signature of eruptions from silicic magma chambers?

New insights into the nature and construction of granitic intrusions provide an opportunity to correlate the rock record of an intrusion to the rock record of volcanism related to the intrusion. We attempt such a correlation between the Silurian Vinalhaven (Maine, USA) intrusive complex and associated volcanic rocks. The largely granitic Vinalhaven intrusion includes a stratigraphic sequence of interlayered gabbro, diorite, and minor granite that preserves field evidence for mingling of coexisting mafic and silicic magmas in a series of ephemeral silicic magma chambers. Within this stratigraphy, blocks of country rocks occur at a number of stratabound horizons in association with mafic sheets. The along-strike lithologic variation of the blocks mirrors the lithologic variation of country rocks along the northern margin of the intrusion, which appears to be the only preserved portion of the intrusions roof. The limited stratigraphic distribution and correlation of lithologies to the roof suggest that the country-rock blocks were stoped during short-lived roof collapse events rather than continuously over the lifetime of the chamber. We suggest that these stratabound roof blocks record collapse of the roof during eruptions from the system, perhaps triggered by the influx of mafic magma, and represent one large caldera-forming eruption followed by a series of smaller eruptions. This interpreted eruptive history of the pluton is consistent with the overall stratigraphy of spatially, temporally, and compositionally related volcanic rocks.

Commingling of magmas in the Bjerkrem-Sogndal lopolith (southwest Norway): evidence for the compositions of residual liquids

A detailed re-examination of some of the more fractionated portions of the Bjerkrem-Sogndal lopolith (BSL) has provided evidence for widespread mixing between quartz mangeritic and monzonoritic (Eia-Rekefjord) magmas. The higher-temperature monzonoritic liquids are preserved as chilled pillows within quartz mangerite. Chemical compositions of these and other fine-grained monzonorites suggest that they are chilled residual liquids related to the BSL, and that they were in equilibrium with the more fractionated cumulates of the BSL. The chilled monzonoritic liquids appear to define a fractionation trend of increasing SiO2 (up to 60 wt. %). The quartz mangerites appear to represent separate partial melts derived from the country rock.