Cathedral Peak Granodiorite

The Cathedral Peak granodiorite is named after its type locality, which is situated in the U.S. Yosemite National Park Cathedral Peak. The Granodioritmassiv is part of the Tuolumne Intrusive Suite ( also Tuolumne Batholith ) - one of the four major Intrusivkomplexe within the Sierra Nevada. Age ranges for the massive fall in the Upper Cretaceous ( Coniacian ) and vary 88-87 million years ago.

Geography

The Cathedral Peak granodiorite is located in eastern California and extends over large parts of Mariposa County and Tuolumne counties, also he touches the Madera County and Mono County. His insights are glaciated, ranging from the higher elevations of Yosemite Valley up to the watershed of the Sierra. At its north end of Tower Peak and Matterhorn Peak are ( with 3743 meters the highest point ), in the southwest part of the Tuolumne Meadows with the Cathedral Peak ( 3326 meters). The Tioga Pass road crosses the southern Granodioritmassiv from east to west. Drainage is generally in the north section to the southwest.

The granodiorite has elongated, rectangular to elliptical dimensions, the longitudinal axis after the south-southeast - north-northwest oriented and about 50 km long. At the northern end it is up to 20 kilometers wide. Its surface area thus reached more than 600 square kilometers - roughly half of the total Tuolumne Intrusive Suite. The solid embraces in its southern section completely the Johnson granite porphyry. For his part, it is to the southeast, surrounded in the southwest and northwest from the Half Dome granodiorite. Along a central constriction, it also touches on the Kuna Crest granodiorite. On the northern and north-eastern side it comes across Paleozoic to Jurassic volcanic and sedimentary rocks niedrigmetamorphe.

Geological Survey

The Cathedral Peak granodiorite is the third and most important at the same magmatic pulse of the Tuolumne Intrusive Suite at the central eastern edge of the Sierra Nevada. The intrusions of igneous sequence extended over a fairly extended period of a total 8.1 million years ago in the Upper Cretaceous. They started in the Turonian 93.5 million years ago and lasted until the beginning of Santoniums before 85.4 million years. Cooling age for the Cathedral Peak granodiorite lie between 88.1 ± 0.2 and 87.0 ± 0.7 million years ago, that fall into the Coniacian.

In about the same time with the Tuolumne Intrusive Suite invaded following intrusive on the eastern edge of the Sierra Nevada batholith:

  • John Muir Intrusive Suite and
  • Mount Whitney Intrusive Suite to the south. As well as the
  • Sonora Plutonic Complex in the north.

The entire surface area of all four Intrusivkomplexe is more than 2500 square kilometers.

The Tuolumne Intrusive Suite in turn is composed of the following Intrusivkörpern ( from young to old):

  • Johnson granite porphyry
  • Cathedral Peak granodiorite
  • Half Dome granodiorite with: porphyritic facies
  • DC -grained facies

Within this Magmenabfolge following trends are generally recognized:

  • A Alterszonierung, with the oldest intrusive bodies, the Kuna granodiorite, at the outer edge and the Last, the Johnson granite porphyry, all inside.
  • An increase in SiO2 and alkali content inside of mafic / intermediate towards more acidic composition.
  • An increase in the content of rubidium inside.
  • A decrease in Al2O3, TiO2, FeO, MgO and CaO content inside.
  • A decrease in the content of barium, strontium and light rare earth such as scandium inward.

Petrographic description

Most important feature of the Cathedral Peak granodiorite porphyry is its structure with numerous, sometimes very large Alkalifeldspatkristallen that can reach up to 20 centimeters in length. The grain sizes in the matrix move by 5 millimeters.

Mineral constituents

Specifically, the Cathedral Peak granodiorite following modal composition comprises:

  • Plagioclase - 47.5 (40 to 50) percent by volume. Idiomorphic to hypidiomorpher, blackboard -like oligoclase with An27 - 29th Twinning after the Carlsbad and Albitgesetz. Shows normal zoning with more calcium - rich core areas ( oligoclase ) and sodium -rich rims ( albite ). Part broken cataclasis and infiltrated by microcline. Ordinary grain size 1-15 mm.
  • Alkali - 20.9 (16 to 25) percent by volume. Perthitischer orthoclase with Or88. Acts as huge as phenocrysts and gusset filling on the base. Grain sizes up to 20 inches occasionally, but usually up to 10 centimeters. Frequency and particle size decrease towards the Johnson granite porphyry. The phenocrysts enclose due to their increased growth rate of other minerals such as biotite, hornblende, plagioclase and alkali feldspar poikilitisch. Sometimes a secondary conversion to clay minerals can be recognized.
  • Quartz - 25.9 volume percent. DC Oversized hypidiomorphe crystals of medium grain size ( 10 mm).
  • Biotite - 3.5 percent by volume. Hypidiomorphe crystals with brown pleochroism.
  • Hornblende - 0.8 percent by volume.
  • Apatite - 0.3 percent by volume. Prisms.
  • Titanite. Most irregular, fine-grained crystals, partly idiomorphic occurring.
  • Opaque ore minerals such as ilmenite and magnetite - 0.6 percent by volume.
  • Accessory minerals are allanite and zircon.
  • Myrmekit in shear zone.

Chemical composition

The following analyzes of Bateman & Chappell, and an average of 18 analyzes ( with scattering region ) of Burgess & Miller to illustrate the chemical composition of the granodiorite:

Compared to an average granodiorite of the Cathedral Peak granodiorite has a higher SiO2 content, he is also richer in alkalis and a member of the shoshonitischen high -K series. He is a normal aluminoses ( metaluminoses ), sodium - fitting rock and belongs to the intrusive, from partial melting of the cladding region emerged I- type. It also forms part of the series and was at kalkalkalischen Subduktionsvorgängen in the root zone of a volcanic island arc.

The trace elements compared to average granodiorite a very strong enrichment of barium and strontium is still visible, nickel and chromium, however, show very low values.

The content of LREE ( light rare earths) is increased, a europium anomaly but not present.

Structures and phenomena

Resulting magmatic conditions structures are:

  • Documents. This is indicated by an accumulation of hornblende and biotite. There are two intersecting systems with the main direction of the south-southeast - north-northwest ( steep standing to 77 °, with generally also steeply angled lineation ) and subordinated to east-southeast - west-northwest.
  • Streaks. General strike to the south-southeast - north-northwest ( 157 N with local deviations up to 50 ° ) at a relatively steep incidence direction ( 60 °) to the east-northeast.
  • Head courses (English ladder dykes ) - tubular, localized puffing smaller magmatic pulses. These structures can be displaced by later movements of magmatic origin.
  • Offsets possible disturbance in still magmatic state, for example in Schlieren ( hanging wall to west southwest, sinistral offset with a slightly oblique component ) or to manager transitions. Just hard to see in the homogeneous parent rock, but also available here. The breaking points are usually filled with aplites or Mikroklinanreicherungen.
  • Inclusions of Mikrogranitoiden. These are similar to the host rock in their mineral composition, but have significantly more mafic minerals such as hornblende and biotite. As phenocrysts of plagioclase and hornblende act with a grain size of 5 to 8 mm. The inclusions are sometimes surrounded by up to 3 inches wide felsic hems. Join sporadically and in swarms, a preferred orientation is not evident.
  • Aplitgänge, usually 1-3 inches wide, fine-grained and homogeneous. By beating all other structures, mostly with sharp contacts. Wider gears can pegmatitic core zones with quartz, plagioclase and alkali feldspar contain. Smaller Fiedergänge end sometimes blurred in the host rock.

Structures of tectonic origin:

  • Repeated Kataklasis: of magmatic plagioclase
  • In minerals of the matrix
  • At the edges of microcline phenocrysts

Structures metasomatic origin, observed in a shear zone on the eastern edge:

  • Myrmekit
  • Replacement of plagioclase by microcline

Overall, all these phenomena point to very complex history of the Cathedral Peak Gtanodiorits, which was composed of igneous, tectonic and metasomatic sections and may be caused by a simultaneous pairwise interaction of these factors.

Formation

So far it was assumed that a single large magma chamber, which then successively the various granitoids secreted by fractional crystallization, including the Cathedral Peak granodiorite towards the end of their existence. This somewhat simplistic model is drawn by the following facts in question:

  • The extremely long lasting 8.1 million years activity of these magma chamber.
  • Inconsistencies in the trace element distribution and the initial isotope ratios of strontium and neodymium.

The isotope ratios suggest rather to the mixing of two Magmentypen - a jacket- like melt and a granitic melt with the composition of the Johnson Granite Porphyry.

Thermobarometric data show a penetration depth of 6 km, and crystallization temperatures for a range of 750 to reduce to 660 degrees Celsius.

In addition, feldspars, hornblende, biotite and magnetite often show signs of separation in the low-temperature Subsolidusbereich.

Equally important is the fact that the Cathedral Peak granodiorite is not always sharply separated from the Half Dome granodiorite, but sometimes shows smooth transitions of more than a hundred meters. Moreover, the two granodiorites overlap in their geochemical parameters, the differences are mainly gefügekundlicher Art The two granodiorites therefore represent a continuum and are no two distinct Intrusivpulse. However, the contact conditions at the Johnson granite porphyry enclosed are sharp.

Controversy is still the mode of origin of the Mikroklins in the shear zone. M.D. Higgins advocated the possibility of recrystallization based on the Ostwald ripening process by metasomatic fluids. LG Collins comes from a erfolgendem below the solidus metasomatic growth ( potassium and silicon - metasomatism ), which was made ​​possible by a continuous, tectonically induced cataclasis. This process requires a cataclastic fracturing of the starting crystals to be fully effective. This is observed in the Cathedral Peak granodiorite in a ductile shear zone on the eastern edge of the massif.

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