Peak Granodiorite Research Articles

On the origin of alkali feldspar megacrysts in granitoids. Part 2: evidence for nucleation and growth under magmatic conditions from crystal size distributions of the Cathedral Peak Granodiorite, California, USA

The mechanisms whereby alkali feldspar megacrysts form have been debated for several decades; yet, we do not understand well the processes that lead to their formation. We take advantage of glacially polished outcrop surfaces from the Cathedral Peak Granodiorite in the Tuolumne Intrusive Complex, CA to quantitatively characterize alkali feldspar textures, to provide better insight into their origin. On the glacially polished surfaces, we traced alkali feldspar crystals > 10 mm in the field. From the same localities, we also collected large slabs and stained them to reveal feldspar textures for crystals < 20 mm in size. We scaned the resulting field tracings and rock slabs to quantify CSDs using image processing techniques with the software ImageJ. The CSDs from glacially polished outcrop surfaces and complementary polished and stained rock slabs reveal two stages of crystallization. Crystals > 20 mm show log-linear CSDs with shallow slopes, suggesting magmatic nucleation and growth on timescales of thousands of years. Crystals < 20 mm define a second stage of crystallization, with much steeper slopes, suggesting a period of enhanced nucleation leading to formation of a groundmass during the final stages of solidification on timescales of decades to centuries. We do not find any evidence for CSDs affected by textural coarsening, or any effects of subsolidus processes. Our data suggest that these megacrysts form in large, slowly cooling magma, where low nucleation rates dominate. These crystals are not special in their magmatic formation—only in their size. A change in solidification conditions led to the formation of a groundmass, which warrants further study to better understand this crystallization stage in a plutonic environment.

Influence of Titaniferous Phases on Tungsten Mineralizing Processes at the Giant Sisson Brook W-Mo Deposit, New Brunswick, Canada: Mineral-Chemical and Geochronological Assessment

The Sisson Brook deposit is a low-grade, large-tonnage W-Mo deposit with notable Cu located in west-central New Brunswick, Canada, and is one of several W-Mo deposits in New Brunswick associated with fluids sourced from granitic plutons emplaced during the Devonian Acadian Orogeny. The younger Devonian-aged stockwork and replacement scheelite-wolframite-molybdenite (and chalcopyrite) mineralization straddles the faulted boundary between Cambro-Ordovician metasedimentary rocks with Ordovician felsic volcaniclastic rocks and the Middle Silurian Howard Peak Granodiorite, with dioritic and gabbroic phases. U-Pb dating of magmatic titanite in the host dioritic phase of the Howard Peak Granodiorite using LA ICP-MS resulted in a 204Pb-corrected concordant age of 432.1 ± 1.9 Ma. Petrologic examination of selected mineralization combined with elemental mapping of vein selvages using micro-XRF and metasomatic titanite and ilmenite grains using LA ICP-MS indicates that saturation of titaniferous phases influenced the distribution of scheelite versus wolframite mineralization by altering the aFe/aCa ratio in mineralizing fluids. Ilmenite saturation in Ti-rich host rocks lowered the relative aFe/aCa and led to the formation of scheelite over wolframite. Altered magmatic titanite and hydrothermal titanite also show increased W and Mo concentrations due to interaction with and/or saturation from mineralizing fluids.

Tabular fracture clusters: Dynamic fracturing produced by volatile expulsion, Sierra Nevada Batholith, California

An undocumented type of fracture system – Tabular Fracture Clusters (TFCs) – occurs in the Cathedral Peak granodiorite, Sierra Nevada Batholith, CA. TFCs are linear zones of sub-parallel, densely spaced fractures, approximately 4–40 cm wide and 3–100 m long. TFCs occur in highest density adjacent to the Johnson granite porphyry, which intruded the Cathedral Peak granodiorite. Individual fractures in TFCs exhibit only opening-mode displacement. Microstructural analysis indicates that TFC fractures contain micro-breccia of angular clasts of host rock. Fine-grained zeolite and quartz are observed along many of the fractures within TFCs, all of which are absent in the host rock. The characteristics of TFCs suggest that they differ from previously documented fracture systems in the Sierra Nevada Batholith. The dense spacing, nature of the fractures, and association of TFCs with miarolitic cavities provide compelling evidence that TFCs formed in response to volatile overpressure from the adjacent Johnson granite porphyry. We attribute the formation of TFCs to dynamic fracturing, based on the clustered nature of the extension fractures and the geometric similarities of TFCs to other dynamic fracturing arrays.

Bedrock fracture control of glacial erosion processes and rates

Research Article| May 01, 2010 Bedrock fracture control of glacial erosion processes and rates Miriam Dühnforth; Miriam Dühnforth 1Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado 80303, USA Search for other works by this author on: GSW Google Scholar Robert S. Anderson; Robert S. Anderson 2Institute of Arctic and Alpine Research (INSTAAR) and the Department of Geological Sciences, University of Colorado, Boulder, Colorado 80303, USA Search for other works by this author on: GSW Google Scholar Dylan Ward; Dylan Ward 2Institute of Arctic and Alpine Research (INSTAAR) and the Department of Geological Sciences, University of Colorado, Boulder, Colorado 80303, USA Search for other works by this author on: GSW Google Scholar Greg M. Stock Greg M. Stock 3Yosemite National Park, Resources Management and Science, El Portal, California 95318, USA Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (5): 423–426. https://doi.org/10.1130/G30576.1 Article history received: 30 Jul 2009 rev-recd: 25 Nov 2009 accepted: 02 Dec 2009 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Miriam Dühnforth, Robert S. Anderson, Dylan Ward, Greg M. Stock; Bedrock fracture control of glacial erosion processes and rates. Geology 2010;; 38 (5): 423–426. doi: https://doi.org/10.1130/G30576.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The importance of rock type and tectonic history on rates of glacial erosion, and the relative roles of glacial quarrying and abrasion, are poorly understood. We use concentrations of cosmogenic 10Be in glacial polish and measurements of bedrock fracture spacing to explore the relationship between erosion rates and rock fracturing at 23 sites along Tuolumne River valley and five sites in Tenaya Canyon in Yosemite National Park, California, USA. Most sites yield 10Be concentrations that can be best explained as reflecting solely postglacial nuclide accumulation. Six sites, however, display anomalously high concentrations, implying incomplete removal of the pre-glacial nuclide inventory during the last glaciation; these require that erosion in the last glacial cycle was <2–3 m. These low-erosion sites occur preferentially in massive units of the Cathedral Peak Granodiorite and the El Capitan Granite. Fractures in high-erosion sites are more closely spaced than in low-erosion sites, with spacings that average 1.1 ± 0.03 m and 3.3 ± 0.1 m, respectively. Our data suggest that the distance between fractures in the rock, dictated in part by the original spacing in a particular pluton, and in part by the specific tectonic history of the Sierra Nevada, governs the pace of glacial erosion in Yosemite. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Growth of complex sheeted zones during recycling of older magmatic units into younger: Sawmill Canyon area, Tuolumne batholith, Sierra Nevada, California

In Sawmill Canyon, located near the eastern margin of the Tuolumne batholith, central Sierra Nevada, California, a series of petrologically and structurally complex, magmatic sheeted zones intrude older granodioritic units (Kuna Crest and equigranular Half Dome) and in one case truncate these units along a sharp contact. These sheeted zones (a) consist of numerous batches of (now frozen) magma, (b) display clear outward growth directions, (c) were actively deforming during and after emplacement resulting in magmatic folds, faults and multiple magmatic mineral fabrics, and (d) are the location of numerous, but localized magma flow structures (schlieren-bounded tubes, troughs, megacryst-rich pipes) and instabilities (load casts, flame structures, slumps, diapirs, ridge and pillar structures). Geochemical data indicate that the sheeted zones largely consist of magmas derived from the Half Dome granodiorite with some late Cathedral Peak granodiorite pulses, and with fractionation and flow sorting forming widespread layering in the above structures. We interpret these sheeted zones to record the pulsing of magma during propagation and expansion of opening-mode (Mode I), submagmatic fractures at the margins of large blocks of older, fairly solidified magmatic pulses that were subsequently removed from the present crustal level. Elsewhere in the Tuolumne batholith we see similar features suggesting that a “recycling” process, i.e., the breaking off of older parts of the magma chamber and incorporation into younger intrusive units, occurred in this batholith. This recycling removed a significant portion of older units and resulted in the formation of sheeted zones and local instabilities in this batholith. Finally this recycling is one process responsible for transfer of zircon crystals between units and for obscuring whole-rock geochemical signatures.

Igneous phenocrystic origin of K-feldspar megacrysts in granitic rocks from the Sierra Nevada batholith

Study of four K-feldspar megacrystic granitic plutons and related dikes in the Sierra Nevada composite batholith indicates that the megacrysts are phenocrysts that grew in contact with granitic melt. Growth to megacrystic sizes was due to repeated replenishment of the magma bodies by fresh granitic melt that maintained temperatures above the solidus for extended time periods and that provided components necessary for K-feldspar growth. These intrusions cooled 89–83 Ma, are the youngest in the range, and represent the culminating magmatic phase of the Sierra Nevada batholith. They are the granodiorite of Topaz Lake, the Cathedral Peak Granodiorite, the Mono Creek Granite, the Whitney Granodiorite, the Johnson Granite Porphyry, and the Golden Bear Dike.

Long-term geochemical variability of the Late Cretaceous Tuolumne Intrusive Suite, central Sierra Nevada, California

Abstract This study investigates the internal anatomy and petrogenesis of the Tuolumne Intrusive Suite (TIS), which comprises metaluminous, high-potassium, calc-alkaline granitoids typical of the Sierra Nevada batholith. Although the TIS has often been cited as an example of a large magma chamber that cooled and fractionated from the margins inward, its geochemistry is inconsistent with closed-system fractionation. Most major elements are highly correlated with SiO 2 , but the scattered nature of trace elements and variations of initial Sr and Nd isotopic ratios indicate that fractional crystallization is not the predominant process responsible for its chemical evolution. Isotopic data suggest mixing between melts of mantle-like rocks and a granitic melt similar in composition to the highest-silica TIS unit. Monte Carlo models of magma mixing confirm that such processes can reproduce the observed variations in major elements, trace elements and isotopic ratios. Thermobarometry suggests emplacement at depths near 6 km and crystallization temperatures ranging from 660 to 750 °C. Feldspars, hornblende, biotite and magnetite exhibit evidence of extensive low-temperature subsolidus exsolution. The TIS as a whole trends toward more evolved isotopic compositions and younger U–Pb zircon ages passing inward. This pattern indicates a general increase in the proportion of felsic, crustally derived melt in the mixing process, which may have resulted from net accumulation of heat added to the lower crust by intrusion of mantle-derived mafic magma. However, the bulk geochemical and isotopic compositions of the equigranular Half Dome Granodiorite, the porphyritic Half Dome Granodiorite and the Cathedral Peak Granodiorite overlap one another and the contacts between them are commonly gradational. We interpret these map units to represent a single petrological continuum rather than distinct intrusive phases. The textural differences that define the units probably reflect thermal evolution of the system rather than distinct intrusive events.

Construction, solidification and internal differentiation of a large felsic arc pluton: Cathedral Peak granodiorite, Sierra Nevada Batholith

Abstract The Tuolumne Batholith (TB), Sierra Nevada Batholith (USA), is an archetypal large, zoned arc intrusion ( c . 1200 km 2 ). Previous work proposed that compositional zonation observed in the TB was produced in-situ by inward differentiation of a large magma chamber and/or large-scale, intrachamber magma mixing. Recent geochronology shows that the TB was intruded over 8–9 Ma, making single pulse fractionation or mixing in a magma chamber of TB dimensions unlikely. We examine processes responsible for compositional variation in the Cathedral Peak Granodiorite, which is the largest mapped unit of the TB. New field, geochemical and geochronological work along a roughly contact-perpendicular 5 km transect indicates: (1) magmatic foliation is steeply-dipping (&gt;60°); (2) field evidence for repeated separation of crystals from melt and local magma mixing is observed; (3) U–Pb zircon ages at opposing ends of the transect are indistinguishable within error ( c . 87.5 Ma); (4) bulk composition varies only modestly but trace elements show variable degrees of scatter; (5) εNd(t) and 87 Sr/ 86 Sr(i) have small variation compared with that in the whole TB. Geochemical and isotopic data are compatible with fractionation of major silicates and accessory minerals. However, the geochemical spatial variation, minor isotopic variation and field evidence suggest that fractionation was highly disorganized and also involved mixing with new input magma and remobilization of crystal mush as the pluton solidified. Our observations are consistent with the construction of a large and dynamic magma system within the last c . 1 Ma of TB growth.

VOLUMINOUS STOPING IN THE MITCHELL PEAK GRANODIORITE, SIERRA NEVADA BATHOLITH, CALIFORNIA, USA

The Mitchell Peak Granodiorite, Sierra Nevada, California, has an outcrop exposure of nearly 50% stoped blocks in its youngest intrusive phase and an estimated 20% exposure of stoped blocks throughout the entire pluton. In this pluton, stoping thus is an important process. It is an excellent locale to study the effect of stoping on the evolution of magmatic systems. Preliminary work in the Mitchell Peak area has shown a complex pattern of stoping at pluton margins, with abundant evidence for mechanical disintegration of the granodiorite blocks. Preserved blocks range in size from hundreds of meters across down to K-feldspar xenocrysts. Many stoped blocks have a sharp contact with the surrounding pluton; rare examples have a diffuse, partially melted contact. Abundant xenocrysts derived from the stoped blocks provide evidence of contamination. In addition, a variety of magmatic features concentrated along the margin of stoped blocks, such as swarms of microgranitoid enclaves, schlieren layering, aplite intrusive bodies and orbicular granodiorite, suggests that these are areas of repeated injection or pulsing of magma. We speculate that repeated injection of magma plays an important role in the mechanical and chemical breakdown of host-rock margins and of stoped blocks. Stoping may play an important role in the thermal, mechanical, and chemical contamination of magmas in arcs, and deserves further attention.

Fabric studies within the Cascade Lake shear zone, Sierra Nevada, California

The Cascade Lake shear zone occurs on the eastern margin of the Tuolumne Intrusive Suite, Sierra Nevada Batholith, California. Foliation in the zone is NNW trending and subvertical, and lineation is moderately south plunging. Deformation is syn-tectonic with emplacement of the Cathedral Peak granodiorite. A deformation gradient exists toward the NE margin of this pluton, with higher strains and lower temperatures of deformation found near the contact. We compare fabric data collected very densely in this shear zone using several techniques: field fabrics, 3D orientation of K-feldspar megacrysts, and AMS (anisotropy of magnetic susceptibility) analysis. In general, the results from the three different methods are in agreement. Deformation in this shear zone is part of a larger pattern of deformation within the Cathedral Peak granodiorite, as recorded by AMS analysis, and dextral shearing associated within the last stage of plutonism within the Sierra Nevada magmatic arc.

Origin of megacrysts in granitoids by textural coarsening: a crystal size distribution (CSD) study of microcline in the Cathedral Peak Granodiorite, Sierra Nevada, California

Abstract Microcline megacrysts in the Cathedral Peak Granodiorite and other parts of the Tuolumne Intrusive Suite were formed by textural coarsening (Ostwald Ripening) of earlier formed crystals. The early-formed crystals nucleated and grew in an environment of increasing undercooling, probably during the ascent of the magma. Emplacement of the magma into warm host rocks promoted textural coarsening. Crystals smaller than a certain size (the critical size) dissolved in the interstitial melt whilst large crystals grew. Microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization. Positive feedback between textural coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts. Megacryst growth was halted when cooling resumed at the end of the intrusive cycle. K-feldspar nucleation was then renewed and K-feldspar crystals grew to form part of the groundmass. It was the particular thermal history of this pluton that promoted textural coarsening—chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture.

Granitic Formations in the East-Central Sierra Nevada Near Bishop, California

This report establishes lithologic units among the granitic rocks of the east-central Sierra Nevada near Bishop, California. In this area the Sierra Nevada batholith is composed chiefly of quartz-bearing plutonic rocks ranging in composition from quartz diorite to alaskite but includes scattered small masses of darker and older plutonic rocks and remnants of metamorphosed sedimentary and volcanic rocks. The granitic rocks are in discrete plutons, either in sharp contact with one another or separated by thin septa of metamorphic or mafic igneous rock or by late aplitic dikes. The granitic rocks are grouped into lithologic units on the basis of composition, texture, and intrusive relations. The units include six new formations, three informal units made up of the rocks in several plutons, and four informal units that include the rocks in single plutons. The new formations are the Inconsolable Granodiorite, Tinemaha Granodiorite, Wheeler Crest Quartz Monzonite, Round Valley Peak Granodiorite, Lamarck Granodiorite, and Tungsten Hills Quartz Monzonite.