Cathedral Peak Granodiorite

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The Matterhorn Peak is made of Cathedral Peak granodiorite

The Cathedral Peak granodiorite was named after its type locality, Cathedral Peak located in the US Yosemite National Park . The granodiorite massif is part of the Tuolumne Intrusive Suite (also Tuolumne Batholith ) - one of the four large intrusive complexes within the Sierra Nevada . Age data for the massif fall in the Upper Cretaceous ( Coniacium ) and vary between 88 and 87 million years.

geography

The Cathedral Range is constructed from Cathedral Peak granodiorite

The Cathedral Peak Granodiorite is located in eastern California and extends over large parts of Mariposa County and Tuolumne counties ; it also touches the Madera County and Mono County . Its outcrops are glaciated and extend from the higher elevations of the Yosemite Valley to the Sierra watershed. At its north end are Tower Peak and Matterhorn Peak (the highest elevation at 3743 meters), and Tuolumne Meadows with Cathedral Peak (3326 meters) in the south-western part . The Tioga pass road crosses the southern granodiorite massif from east to west. In the northern section, drainage generally takes place to the southwest.

The granodiorite has elongated, rectangular to elliptical dimensions, with the longitudinal axis oriented south-southeast-north-northwest and measuring about 50 kilometers. At the northern end it is a maximum of 20 kilometers wide. Its surface area thus reaches a good 600 square kilometers - about half of the entire Tuolumne Intrusive Suite. The south section of the massif completely encloses the Johnson granite porphyry . In turn, it is surrounded by Half-Dome granodiorite to the southeast, southwest and northwest . It also touches the kuna crest granodiorite along a central constriction . On its north and north-east side it encounters Paleozoic to Jurassic low-metamorphic volcanic rocks and sedimentary rocks .

Geological overview

Yosemite National Park Geological Map . Cathedral Peak granodiorite edged in white

The Cathedral Peak granodiorite is the third and at the same time most important igneous pulse of the Tuolumne Intrusive Suite on the central eastern edge of the Sierra Nevada . The intrusions of this magmatic sequence extended over a fairly extended period totaling 8.1 million years in the Upper Cretaceous. They began in the Turonian 93.5 million years ago and lasted until the beginning of the Santonian 85.4 million years ago. Cooling ages for Cathedral Peak granodiorite are between 88.1 ± 0.2 and 87.0 ± 0.7 million years, i.e. fall into the Coniacian .

At about the same time as the Tuolumne Intrusive Suite, the following intrusive bodies appeared on the eastern edge of the Sierra Nevada batholith :

The total surface area of ​​all four intrusive complexes is more than 2500 square kilometers.

The Tuolumne Intrusive Suite is made up of the following intrusive bodies (from young to old):

The following trends can generally be recognized within this magma sequence:

  • An age zone, with the oldest intrusive body, the Kuna granodiorite, on the outer edge and the youngest, the Johnson granite porphyry, on the inside.
  • An inward increase in SiO 2 and alkali content, from mafic / intermediate to more acidic.
  • An inward increase in rubidium content .
  • A decrease in Al 2 O 3 , TiO 2 , FeO, MgO and CaO content inwards.
  • A decrease in the content of barium , strontium and light rare earths such as scandium inwards.

Petrographic description

The most important feature of Cathedral Peak granodiorite is its porphyry structure with numerous, in some cases very large, alkali feldspar crystals that can be up to 20 centimeters long. The grain sizes in the base mass are around 5 millimeters.

Mineral inventory

In particular, the Cathedral Peak granodiorite contains the following modal composition:

  • Plagioclase - 47.5 (40 to 50) percent by volume. Idiomorphic to hypidiomorphic, tabular oligoclase with An 27-29 . Twinning according to the Karlsbad and Albit Act. Shows normal zonal structure with more calcium- rich core areas (oligoclase) and sodium- rich edges ( albite ). Partly broken cataclastically and infiltrated by microcline. Common grain sizes 1–15 millimeters.
  • Alkali feldspar - 20.9 (16 to 25) percent by volume. Perthitic orthoclase with Or 88 . Occurs as giant phenocrystals and as a fillet in the base mass . Grain sizes occasionally up to 20 centimeters, but mostly up to 10 centimeters. Frequency and grain sizes decrease in the direction of the Johnson granite porphyry. Due to their increased growth rate, the phenocrystals enclose other minerals such as biotite, hornblende, plagioclase and alkali feldspar poikilitically. Sometimes a secondary conversion to clay minerals can be seen.
  • Quartz - 25.9 percent by volume. Equally dimensioned hypidiomorphic crystals of medium grain size (10 millimeters).
  • Biotite - 3.5 percent by volume. Hypidiomorphic crystals with brown pleochroism.
  • Hornblende - 0.8 percent by volume.
  • Apatite - 0.3 percent by volume. Prisms.
  • Titanite . Mostly irregular, fine-grained crystals, sometimes also occurring idiomorphically.
  • opaque ore minerals such as ilmenite and magnetite - 0.6 percent by volume.
  • Accessories are allanite and zircon .
  • Myrmecite , in shear zone.

Chemical composition

The following analyzes by Bateman & Chappell and an average value from 18 analyzes (with range) by Burgess & Miller are intended to clarify the chemical composition of the granodiorite:

Oxide
wt.%		 Bateman & Chappell
Burgess & Miller average		 CIPW norm
percent		 Bateman & Chappell average Trace elements
ppm
Burgess & Miller average
SiO 2 69.60 70.29 (67.0-72.0) Q 24.52 25.58 Pb 17.5 (15-20)
TiO 2 0.38 0.41 (0.3-0.6) Or 21.67 20.64 Cu 4.9 (3.2-6.9)
Al 2 O 3 15.34 15.37 (15.0-16.5) From 36.79 35.81 Ni 3.0 (0.7-6)
Fe 2 O 3 1.30 1.40 On 11.85 12.57 Cr 3.3 (0-24)
FeO 0.95 1.03 Tuesday 0.57 0.37 V 41.4 (23-50)
MnO 0.06 0.06 (0.5-0.8) Hy 1.63 1.82 Zr 135.9 (82-165)
MgO 0.70 0.72 (0.6-0.9) Mt 1.87 2.01 Y 8.3 (4.9-11)
CaO 2.68 2.82 (2.2-3.2) Il 0.73 0.77 Sr 633.2 (487-758)
Na 2 O 4.31 4.24 (4.0-4.5) Ap 0.32 0.36 Ba 748.0 (410-1182)
K 2 O 3.64 3.50 (2.8-4.2) Rb 132.5 (114-166)
P 2 O 5 0.14 0.16 (0.12-0.20) Nb 7.8 (4.9-10)
Mg # 0.55 0.54 Sc 3.6 (1.7-4.5)
A '/ F 0.08 0.11 Ga 20.9 (19-23)
Al / K + Na + Ca 0.96 0.97 Zn 57.8 (38-65)

Compared to an average granodiorite, Cathedral Peak granodiorite has an increased SiO 2 content, it is also richer in alkalis and a member of the Shoshonite high K series . It is a normal aluminoses (metaluminoses), sodium -concentrated rock and belongs to the intrusive I-type that emerged from partial melting of the mantle area . It also forms part of the calcareous series and was created during subduction processes in the root area of ​​a volcanic island arc .

In the case of trace elements , a very high concentration of barium and strontium can still be seen compared to average granodiorites, while nickel and chromium show very low values.

The LREE (light rare earths) content is increased, but there is no europium anomaly .

Structures and phenomena

Structures created under magmatic conditions are:

  • Locations . To be recognized by an enrichment of hornblende and biotite. There are two intersecting systems with the main direction south-south-east-north-north-west (steep, at 77 °, with generally also steep lineation ) and subordinate east-south-east-west-north-west.
  • Streaks . General strike direction south-southeast-north-northwest (N 157 with local deviations of up to 50 °) with a relatively steep direction of incidence (around 60 °) to east-northeast.
  • Conductor passages ( Engl. Ladder dykes ) - tubular, localized puffing smaller magmatic pulses. These structures can be displaced by later movements of magmatic origin.
  • Offsets at faults in the still magmatic state, for example at streaks (hanging wall to the west-south-west, sinistral offset with slightly inclined components) or at ladder corridors. Difficult to recognize in the homogeneous parent rock, but also present here. The breakpoints are usually filled with aplites or microcline enrichments.
  • Inclusions of microgranitoids. In terms of their mineral composition, these are similar to the host rock, but have significantly more mafic minerals such as hornblende and biotite. Plagioclase and hornblende with a grain size of 5 to 8 millimeters act as phenocrystals. The inclusions are sometimes surrounded by rocky fringes up to 3 centimeters wide. Occur sporadically and in swarms, a preferred orientation is not recognizable.
  • Aplit tunnels , usually 1–3 centimeters wide, fine-grained and homogeneous. Penetrate all other structures, mostly with sharp contacts. Wider corridors can contain pegmatitic core zones with quartz, plagioclase and alkali feldspar. Smaller pinnacles sometimes end up blurred in the adjacent rock.

Structures of tectonic origin:

  • Repeated cataclasis :
    • of igneous plagioclases
    • of minerals in the matrix
    • at the edges of the microcline phenocrystals

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

  • Myrmecite
  • Substitution of plagioclase by microcline

Overall, all these phenomena reveal a very complex history of the origin of the Cathedral Peak Gtanodiorite, which is composed of igneous, tectonic and metasomatic sections and was possibly caused by a simultaneous paired interaction of these factors.

Emergence

Cataclastically broken plagioclase, twinned according to the albite law, is infiltrated by microcline

So far, a single large magma chamber has been assumed, which then successively separated the various granitoids through fractional crystallization , including the Cathedral Peak granodiorite towards the end of its existence. This somewhat simplistic model is called into question by the following facts:

Rather, the isotope ratios indicate the mixing of two types of magnesia - a mantle- like melt and a granitic melt with the composition of Johnson granite porphyry.

Thermobarometric data show a penetration depth of 6 kilometers and crystallization temperatures over a range of 750 down to 660 degrees Celsius.

In addition, feldspars, hornblende, biotite and magnetite often show signs of segregation in the low-temperature subsolidus area .

Also of importance is the fact that the Cathedral Peak granodiorite cannot always be sharply separated from the half dome granodiorite, but sometimes shows smooth transitions of more than a hundred meters. In addition, the two granodiorites overlap in their geochemical parameters, the differences are mainly of a structural nature. The two granodiorites therefore represent a continuum and not two intrusive pulses that are different from each other. However, the contact relationships with the enclosed Johnson granite porphyry are sharp.

The mode of formation of the microcline in the shear zone is still controversial. MD Higgins advocated the possibility of recrystallization based on the Ostwald ripening process using metasomatic fluids. LG Collins assumes metasomatic growth below the solidus (potassium and silicon metasomatosis), which was made possible by a continuous, tectonically caused cataclase. This process requires a cataclastic breaking of the starting crystals in order to be fully effective. This can be observed in Cathedral Peak granodiorite in a ductile shear zone on the eastern edge of the massif.

Individual evidence

  1. ^ Bateman, PC & Chappell, BW (1979). Crystallization, fractionation and solidification of the Tuolumne intrusive series. Yosemite National Park, California. Geological Society of America Bulletin, 90: 465-482
  2. ↑ Coleman, DS, Gray, W. & Glazner, AF (2004). Rethinking the emplacement and evolution of zoned plutons: geochronologic eviden ce for incremental assembly of the Tuolumne Intrusive Suite, California. Geology, 32, 433-436.
  3. ↑ Kistler, RW, Chappell, BW, Peck, DL & Bateman, PC (1986). Isotopic variation in the Tuolumne intrusive suite, central Sierra Nevada, California. Contributions to Mineralogy and Petrology, 94, 205-220.
  4. ↑ Gray, W., Glazner, AF, Coleman, DS & Bartley, JM (2008). Long-term geochemical variability of the Late Cretaceous Tuolumne Intrusive Suite, central Sierra Nevada, California. In: Annen, C. & Zellmer, GF Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geological Society Special Publication 304.
  5. ^ Titus, SJ, Clark, R. & Tikoff, B. (2005). Geologic and geophysical investigation of two fine-grained granites, Sierra Nevada Batholith, California; evidence for structural controls on emplacement and volcanism. Geological Society of America Bulletin, 117, 1256-1271.
  6. ^ Higgins, MD (1999). Ostwald ripening: in Understanding Granites: Integrating Modern and Classical Techniques, Castro, A., Fernandez, C. and Vigneresse, JL, (editors), Special Publication 168, Geological Society of London, London, p. 207-219.
  7. ^ Collins, LG and Collins, BJ (2002). K-metasomatism of plagioclase to produce microcline megacrysts in the Cathedral Peak granodiorite, Sierra Nevada, California, USA

swell

  • Burgess, SD & Miller, JS (2008). Construction, solidification and internal differentiation of a large felsic arc pluton: Cathedral Peak Granodiorite, Sierra Nevada Batholith. In: Annen, C. & Zellmer, GF Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geological Society Special Publication 304.
  • Collins, LG and Collins, BJ (2002). K-metasomatism of plagioclase to produce microcline megacrysts in the Cathedral Peak granodiorite, Sierra Nevada, California, USA. ISSN  1526-5757 , electronic Internet publication, no.41 .
  • Gray, W., Glazner, AF, Coleman, DS & Bartley, JM (2008). Long-term geochemical variability of the Late Cretaceous Tuolumne Intrusive Suite, central Sierra Nevada, California. In: Annen, C. & Zellmer, GF Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geological Society Special Publication 304.

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