Sven Morgan

Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study

The Papoose Flat pluton of eastern California is commonly cited in the geologic literature as a classic example of a “forcibly” emplaced pluton, although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial. Reexamination of this Late Cretaceous (83 Ma) pluton, using a combination of new structural, petrographic, microstructural, and anisotropy of magnetic susceptibility (AMS) data on the pluton–wall-rock system, has shown that the pluton is an inclined and internally zoned tabular structure, assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12–15 km. Initial pluton formation involved magma ascent in a vertical west- northwest–striking feeder dike, which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying Cambrian metasedimentary rocks, leading to formation of a southwest-dipping sill. Subsequent sill inflation, accompanied by horizontal infilling from the feeder dike at the base of the sill, resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof, facilitated by thermal weakening as the wall-rock temperatures progressively rose during emplacement of successive magma pulses. Cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton. We have been unable to document any regional-scale structures (e.g., of equivalence to similar-age synplutonic strike-slip shear zones in the Sierra Nevada batholith to the west) that may have controlled emplacement of the Papoose Flat pluton. However, this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time; indeed, tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level. Simple thermal modeling, using microstructural and thermobarometric data, indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr. This rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional- scale deformation.

Laccolith-like emplacement model for the Papoose Flat pluton based on porphyroblast-matrix analysis

Detailed porphyroblast-matrix analysis within the concordant metasedimentary aureole rocks surrounding the Papoose Flat pluton of eastern California indicates that inclusion trails within porphyroblasts can be used as strain markers to restore the aureole rocks to their prepluton emplacement position. Using porphyroblast-matrix relationships in combination with measurement of stratigraphic sections and whole-rock geochemical analyses, we have determined the kinematics of rotation, the change in thickness and volume, and the amount of translation of the metasedimentary formations within the aureole. These data are consistent with initial emplacement of the magma as an inclined sill and subsequent inflation into a pluton or laccolith. The combination of structural and porphyroblast-matrix analysis leads to a three-dimensional kinematic history of the wall rocks wherein vertical upward translation represents a significant part of the pluton emplacement-related strain history.

An Overview of Paleozoic-Mesozoic Structures Developed in the Central White-Inyo Range, Eastern California

The central White-Inyo Range of eastern California is composed of a 7.5 km thick succession of Neoproterozoic to Upper Cambrian sedimentary rocks that have been deformed into a series of NE and N to NW-trending folds. The map distribution of the sedimentary rocks is primarily controlled by the younger N to NW-trending folds that define the White Mountain-Inyo anticlinorium. Smaller-scale folds and associated cleavage, which strongly fans in a divergent pattern about the fold hinge surfaces, are associated with both periods of fold development. Older NE-trending folds and associated cleavage are developed in a NW-trending belt that crosses the range in a structural saddle between the White Mountain and Inyo anticlines. These NE-trending folds are of smaller wavelength than the older N to NW-trending regional-scale folds, although both fold generations are developed on the cm-km scale. The intensity of both fold and cleavage development generally increases toward the west. Interference between the NE and th...

Metamorphism and fluid flow in the contact aureole of the Eureka Valley–Joshua Flat–Beer Creek pluton, California

We examined the petrology and stable isotope compositions of metamorphic rocks in the thermal aureole of the Eureka Valley‐ Joshua Flat‐Beer Creek (EJB) composite pluton, White Mountains, eastern California, to determine the physical conditions of contact metamorphism and associated fl uid fl ow. The Jurassic EJB pluton is one of several bodies intruded during protracted Mesozoic magmatism in the White-Inyo Mountains. Emplacement of the pluton rotated and stretched the intruded greenschist-facies Cambrian sedimentary rocks to vertical orientations that are subparallel to the pluton contact. Traverses across the contact aureole included marbles, calc-silicates, schists, and quartzites. Each lithology behaved differently during contact metamorphism. Most dolomite and calcite marbles did not equilibrate with an external fl uid as revealed by original, unshifted δ 18 O and δ 13 C values of 20‰‐25‰ (relative to standard mean ocean water [SMOW]) and ‐1‰ to +2‰ (relative to PeeDee belemnite [PDB]), respectively. Largest isotopic shifts occurred in calc-silicates where δ 18 O decreased to ~15‰. The moderate oxygen-isotope ratio shift suggests that the calc-silicates equilibrated with an aqueous fl uid, magmatic and/or metamorphic, whose oxygen-isotope ratio was buffered by the surrounding silicate metamorphic rocks. Infi ltration of the calc-silicates by an aqueous fl uid in the high-grade part of the aureole is also demonstrated by the presence of grossular garnet and vesuvianite. Schists in the aureole vary in mineralogy and compositions. Harkless and Saline Valley formations have higher Al/K ratios that resulted in more abundant muscovite and andalusite, than the Campito Formation, which contains ubiquitous K-feldspar. On the western side of the pluton, partial melting of the Harkless Formation was driven by fl ow of aqueous fl uids that was focused between the pluton and marbles more distant from the contact. Metamorphic pressure-temperature (P-T) pseudosections for the schists suggest that partial melting has occurred between 2.5 and 3 kbar and >660 °C. The pseudosections also predict a narrow subsolidus cordierite fi eld. However, cordierite , feldspars, and other silicate minerals were altered by pervasive fl uid fl ow through the schists below 500 °C. This fl uid fl ow likely was driven by slow cooling of the large EJB pluton and/or continued magmatism in the White-Inyo Mountains, particularly during later emplacement of the Birch Creek pluton to the west of the EJB pluton.

Magma sheets defined with magnetic susceptibility in the Maiden Creek sill, Henry Mountains, Utah, USA

In the ∼20-m-thick Maiden Creek sill of the Henry Mountains (Utah, USA) intrusive complex, two magma sheets are locally separated by a 1.5-m-thick lens of sandstone. We studied the boundary between these sheets at the termination of this sandstone lens, where the upper sheet directly overlies the lower sheet, in order to test the reliability of using magnetic susceptibility in delineating internal magmatic contacts. The contact between these two sheets is along a cliff face and defined by a thin (<1 cm) brittle-ductile shear zone. Measurements of magnetic susceptibility (K) were collected within a grid every 20 cm across this contact. Drill cores (72) were also collected along four traverses across the shear zone. Mapping K across the cliff face reveals an abrupt decrease immediately below the shear zone contact. 1 m below the contact, K unexpectedly increases again to the same levels observed above the contact. This lower boundary coincides with a 1–2-mm-thick minor fracture zone. The 1-m-thick low-K zone (LKZ) is characterized by more intense microfracturing and is bleached compared to the surrounding igneous rock. Plotting the magnetic foliation from the drill cores reveals abrupt changes to the orientation across both the shear zone and fracture zone. We hypothesize that the LKZ was the original magma sheet that intruded the sandstone. The high-K zones above and below the LKZ represent later sheets that intruded above and below the original sheet, fracturing the partially or wholly crystallized original intrusion. These later sheets exsolved fluids that were injected into the original sheet, resulting in more advanced oxidation of magnetite and thus lowering the K. Alternatively, it is possible that the LKZ is simply the altered zone at the top of a thicker older sheet that was modified by the intrusion of a younger overlying sheet.

Physical Geology of High‐Level Magmatic Systems

The title of this volume almost says it all. This book is a compilation of fifteen papers on upper crustal intrusions that were presented in a workshop on laccoliths and sills (types of magma bodies) in Freiburg, Germany, in October 2002. The papers can be divided into two categories: 10 detailed descriptions of shallow-level magmatic systems, mostly from eastern and southern Europe, and five shorter papers on modeling and seismic studies of sills, laccoliths, and dikes.