Rachel J. Beane

Zircon record of the plutonic-volcanic connection and protracted rhyolite melt evolution

The potential petrogenetic link between a crystal-poor rhyolite (the Rhyolite Canyon Tuff) and its associated subvolcanic intrusion and crystal-rich post-caldera lavas from Turkey Creek, Arizona (USA), is examined using zircon chemical abrasion–thermal ionization mass spectrometry U-Pb geochronology and inductively coupled plasma mass spectrometry trace element analyses. U-Pb ages indicate that zircon growth within the rhyolite and the dacite-monzonite porphyry magmas was coeval over ∼300 k.y. prior to the large eruptive event. Trends in zircon trace elements (Hf, Y/Dy, Sm/Yb, Eu/Eu*) through time in the dacitic-monzonitic units and rhyolite reflect melt evolution dominated by crystal fractionation. Importantly, the Y/Dy ratio in zircons in both units remains mostly similar for the first ∼150 k.y. of the system’s evolution, but the dominant population in the rhyolitic unit diverges from that of the dacite-monzonite porphyry ∼150 k.y. before eruption. We interpret this divergence in trace element composition to record the assembly time of the melt-rich cap within its intermediate mush zone in the upper crustal reservoir. These results are consistent with (1) a connection between plutonic and volcanic realms in the upper crust, (2) a protracted time scale for constructing an intermediate mush large enough to hold 500 km 3 of rhyolite, and (3) the prolonged extraction of that melt prior to eruption.

Metasomatism in Serpentinite Mélange Rocks from the High-Pressure Maksyutov Complex, Southern Ural Mountains, Russia

Metasomatized basalts are enclosed in antigorite serpentine mélange of the Upper Unit of the Maksyutov Complex, southern Ural Mountains, Russia. The structurally Lower Unit of this complex consists predominantly of mafic eclogite and sedimentary gneiss metamorphosed at ultrahigh pressures; rocks of the Upper Unit underwent blueschist-facies conditions. The metasomatized rocks are fine-grained blocks, variably composed of chlorite, garnet, lawsonite, white-mica, epidote, and titanite. Partial rodingitization accompanied serpentinization of the surrounding peridotite, and predated the high-pressure metamorphism that led to the formation of lawsonite. Extensive Mg metasomatism followed rodingitization, leading to prevalent Mg-rich rims around the mafic blocks. Potassium metasomatism, although minor compared to the rodingitization and Mg metasomatism, contributed to the formation of muscovite pseudomorphs after lawsonite during exhumation.

Formation of rhyolite at the Okataina Volcanic Complex, New Zealand: New insights from analysis of quartz clusters in plutonic lithics

Abstract Granitoid lithic clasts from the 0.7 ka Kaharoa eruption at the Tarawera volcano (Okataina Volcanic Complex, Taupo Volcanic Zone, New Zealand) provide insight into the processes of rhyolite formation. The plutonic lithic clasts of the Kaharoa eruption consist of (1) quartz phenocrysts, which are often grouped into clusters of two to eight quartz grains, (2) plagioclase phenocrysts (mostly ~An40 with up to An60 cores), and (3) interstitial alkali feldspar. Quartz orientations obtained through electron backscatter diffraction (EBSD) methods show that 78% of the 82 analyzed clusters have at least one pair of quartz grains with the dominant dipyramidal faces matched. Variations in cathodoluminescence (CL) zoning patterns of the quartz suggest that quartz clusters came together after initial crystal growth and that many quartz crystals were subject to one or more resorption events. The process of quartz crystals with different magmatic histories coming together into common relative orientations to form clusters is indicative of oriented quartz synneusis and suggests a history of crystal accumulation. The quartz clusters are interpreted to have formed as part of a crystal cumulate mush within a shallow magma chamber where quartz crystals rotated into contact along their dominant dipyramidal faces during hindered settling and/or compaction. The preservation of oriented quartz clusters from the Kaharoa plutonic lithics thus provides evidence for synchronous, shallow pluton formation from a cumulate mush during active silicic volcanism. This result is consistent with models whereby meltrich, high-silica rhyolite formation occurs via interstitial melt extraction from a low-silica rhyolite mush in the shallow crust.

Origin of quartz clusters in Vinalhaven granite and porphyry, coastal Maine

We use the crystallographic orientations of quartz crystals, as determined with EBSD, to provide new evidence for the formation of clustered quartz crystals during magma crystallization. Vinalhaven is dominated by granite, with minor porphyry that formed when granite remelted during input of coeval basalt. CL zoning suggests that most quartz clusters in granite and porphyry formed by synneusis, the “swimming together” of preformed crystals. In granite, most quartz pairs in clusters have random orientations—only about 10% have parallel or Esterel twin orientations. Porphyry has fewer quartz clusters, and all pairs have approximately parallel or Esterel twin orientations. CL zoning of quartz pairs in porphyry indicates that they attached prior to a major remelting event. Interpretation of the Vinalhaven quartz clusters leads us to propose that oriented synneusis occurs during crystal accumulation on a magma chamber floor. During hindered settling, some quartz crystals should have come into contact along their dipyramidal faces. Once in contact, continued settling and loss of interstitial melt may have rotated some quartz crystals such that lattices on their dipyramidal faces matched—producing parallel and Esterel twin orientations and creating strong bonds between pairs. Only a small proportion of pairs with matched dipyramidal faces formed in the granite and, during rejuvenation to produce porphyry, only these oriented pairs survived. Hence, the presence of oriented synneusis in a plutonic rock may demonstrate a history of crystal accumulation.

Using the Scanning Electron Microscope for Discovery Based Learning in Undergraduate Courses

Laboratory exercises incorporating a Scanning Electron Microscope (SEM) encourage undergraduate students to explore geologic concepts and methods. A variable pressure (VP) SEM, with attached Energy Dispersive Spectrometer (EDS) and Electron Backscatter Diffractometer (EBSD), has been used in introductory, mineralogy, petrology and structural geology courses to examine sand morphology, quantify mineral chemistry, estimate the temperature of metamorphism, identify microfossils, and pursue student-designed research questions. Student response to using the SEM has been overwhelmingly positive. The undergraduate students, regardless of course level, tend to master the basics of operating the SEM quickly. After a half hour (or so), most students feel confident enough to take control of the SEM and use it to answer questions, or test ideas. Laboratory exercises are designed to allow students some freedom to pursue their own ideas and hypotheses within the framework of a broader geologic question or concept.

Chopinite-sarcopside solid solution, [(Mg,Fe)3□](PO4)2, in GRA95209, a transitional acapulcoite: Implications for phosphate genesis in meteorites

Abstract Orthophosphate, (Mg,Fe,Mn)3(PO4)2 with XMg = Mg/(Mg+Fe) = 0-0.89 and Mn/Fe = 0.05-0.3 and chladniite-johnsomervilleite, MnNa8(Ca4Na4)(Mg,Fe,Mn)43(PO4)36 with XMg = 0.44-0.81 and Mn/Fe = 0.3-0.8, are minor constituents of meteorite Graves Nunataks (GRA) 95209, a transitional acapulcoite consisting mostly of forsterite (Fa7) and enstatite (Wo3Fs7-8) with subordinate clinopyroxene (Wo41-45 Fs4-6) and plagioclase (Or1-2An10-19), and cut by Fe,Ni metal veins. Electron backscatter diffraction patterns and maps, together with chemical analyses and Fe-Mg-Mn distribution among phosphates, confirm identification of the orthophosphate as sarcopside, chopinite, and farringtonite; no graftonite was found. Phosphates are found as (1) narrow rims between metal and forsterite or orthopyroxene; (2) aggregates having the same outline as metal; and (3) inclusions and stringers in metal, including a ring around a graphite rosette. Electron microprobe analyses of sarcopside/chopinite-johnsomervilleite/ chladniite pairs give a regular Fe-Mg distribution with KD = (Mg/Fe)Src/Chp/(Mg/Fe)Jhn/Cld = 0.584 consistent with terrestrial sarcopside-johnsomervilleite pairs, whereas analyses of farringtonite-chladniite pairs give KD = 1.51, but the Mg-Fe distribution is less regular. Textural relations suggest that Fe-Mn sarcopside originally formed by oxidation of P in metal and replacement of the metal and, through interaction with silicates, was converted to magnesian sarcopside-chopinite and farringtonite, i.e., the silicate matrix acted as a reservoir of Mg that could be exchanged with Fe and Mn in the sarcopside. Using the farringtonite-chopinite univariant curve determined in hydrothermal experiments by F. Brunet and others, isopleths calculated for the most magnesian chopinite in GRA95209, XMg = 0.65, give 4-7 kbar at 500-1100 °C, pressures far too high for the acapulcoite-lodranite parent body. Two scenarios could explain the discrepancy: (1) chopinite and magnesian sarcopside persisted metastably into the farringtonite stability field as Mg-Fe exchange progressed and the source volume for GRA95209 cooled; (2) a very mild shock event was intense enough to convert Fe-rich farringtonite (XFe = 0.4-0.6) to magnesian sarcopside and chopinite, but not enough to deform olivine in the source volume. Whether metastability could have played a role in chopinite formation would best be answered by experiments on the Mg3(PO4)2-Fe3(PO4)2 system under anhydrous conditions. If the transformation was found to be as kinetically fast as in the hydrothermal experiments, then shock would become the more plausible explanation for the presence of chopinite in this meteorite.

Protolith Signatures and Element Mobility of the Maksyutov Complex Subducted Slab, Southern Ural Mountains, Russia

Geochemical data from the Maksyutov Complex in the southern Ural Mountains provide evidence for the nature of the protoliths of eclogite boudins and sedimentary gneiss from the ultrahigh-pressure metamorphosed (UHPM) subducted slab, and for an evaluation of fluid-mediated mass transfer in the Devonian to Carboniferous subduction zone. Eclogites have major- and trace-element compositions of tholeiitic basalts with enriched mid-ocean ridge basalt (E-MORB) characteristics, whereas sedimentary gneiss resembles continent-derived shales. The geochemical data combined with field relations and Proterozoic protolith ages suggest the eclogite boudins formed as dikes or sills of basalt in sedimentary continental material of the East European platform during the initial opening of the paleo-Uralian ocean. Portions of the mafic boudins show large-ion lithophileelement (LILE) and light-rare-earth element (LREE) enrichment. Fluid transport of the LILE and LREE in the subduction zone seems to have been channelized (or otherwised spatially organized) on a scale of meters, rather than resulting from pervasive fluid flow. The metasomatizing fluids were most likely dehydration fluids produced during prograde metamorphism of sediments or K-altered basalts that were preserved through peak metamorphism and may have transported additional elements during retrograde metasomatism.

Petrogenesis of the Sugarloaf syenite, Pikes Peak batholith, Colorado

Sugarloaf Peak is one of seven sodic plutons that lie within or adjacent to the ca. 1.08-Ga Pikes Peak batholith in central Colorado. This report represents the first study of the Sugarloaf pluton. Major element and modal analyses from the other six plutons (Lake George, Tarryall, Rampart Range, West Creek, Mt. Rosa, and Spring Creek), together with data presented here, indicate that sodic and potassic rocks from all of them were produced by fractional crystallization of mantle-derived basaltic magmas. The Sugarloaf pluton is composed of fine-grained, medium-grained, coarse-grained, and pegmatitic syenite. The syenites lack quartz and are dominated by perthitic feldspar and ferrorichterite amphibole. The fine-grained syenite intrudes the medium- and coarse-grained syenites. The Sugarloaf pluton is surrounded by coarse-grained Pikes Peak Granite, which is the predominant rock type in the batholith. The linear arrangement of six of the seven sodic plutons parallel to major Precambrian fault trends suggests that the emplacement of Sugarloaf pluton may be rift-related. The Sugarloaf syenites have high total alkalis, high FeO (total), low CaO, and low MgO concentrations. They are also enriched in rare earth elements (REE) and high field strength elements (HFSE). Pronounced trace element variation among the Sugarloaf syenites can be explained partially by models of fractional crystallization. A plot of Ba versus Sr shows that compositions of the syenites closely follow modeled fractionation vectors for potassium feldspar. The fractional crystallization trends show that fine-grained syenite is the most chemically evolved, consistent with field relations that show the fine-grained syenite intruded the medium- and coarse-grained syenites. Accessory mineral fractionation, release of volatiles, or removal of pegmatitic fluids also may have influenced geochemical variations among the Sugarloaf syenites.

A new view of an old suture zone: Evidence for sinistral transpression in the Cheyenne belt

The type locality of the Archean–Paleoproterozoic suture zone in the southern Rocky Mountains is marked by a series of subvertical shear zones collectively called the Cheyenne belt. The Cheyenne belt is a key structure for developing models for 1780–1740 Ma tectonism along the southern margin of the Archean Wyoming Province, which heralded a rapid period of continental amalgamation. This paper tests existing structural and plate-tectonic models for the Cheyenne belt with detailed geologic mapping, kinematic analyses, quartz crystallographic fabric analyses, and deformation mechanism analyses of the northern mylonite zone of the eastern Medicine Bow Mountains. Mylonites of this zone record a complex deformation history, but the main deformation phase was sinistral/northwest-side-up oblique transpression. Evidence for southeast-side-up, dip-slip motion that characterizes many other areas of the belt is confined to ultramylonites immediately adjacent to the terrane boundary. Hence, fabrics related to sinistral transpression were likely overprinted by southeast-side-up motion. Sinistral strike-slip motion is recorded in at least two other localities in the Cheyenne belt. Because synmetamorphic fabrics on both sides of the suture zone record sinistral strike-slip and northwest-side-up motion, this was probably the dominant deformation style in the field area and may have been the dominant deformation style throughout the Cheyenne belt. Based on these data and regional constraints, we interpret the Cheyenne belt as a subvertical transpressional stretching fault system that simultaneously accommodated sinistral strike-slip motion, penetrative horizontal shortening, and dip-slip motion related to differential crustal thickening between the relatively cold Wyoming Province and younger, hotter rocks to the south.