English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

The pre-Cambrian rocks in the Northgate district on the western flank of the Medicine Bow Mountains, Colorado, consist of a gneiss, complex invaded by a granitic stock. A few dacite porphyry dikes cut the gneiss complex and are older than the stock. Dynamothermal metamorphism converted the parent rocks of the gneiss complex into a hornblende-plagioclase gneiss (hornblende gneiss), and was closely followed by widespread metasomatic transformations. Alkali and silica metasomatism converted relatively large masses of hornblende gneiss to quartz monzonite gneiss in the northern and southeastern parts of the district; smaller bodies of quartz monzonite gneiss were formed through the central part of the district where they are associated with abundant pegmatite of related metasomatic origin. Biotite-garnet-quartz-plagioclase gneiss (biotite-garnet gneiss) is a common associate of the pegmatite and was formed by local "basic" transformation of hornblende gneiss in a zone peripheral to the main zone of alkali metaspmatism. Hydrothermal metamorphism of small ultramafic bodies developed a number of zoned deposits of serpentine, chlorite, tremolite-actinolite, and vermiculite. Shearing during and after granitization reduced much of the rock in the gneiss complex to mylonite along an eastwardtrending zone through the south-central part of the district. Farther north, where shearing was less intense, only hornblende gneiss was much affected, and irregular bodies of hornblendebiotite-quartz-plagioclase gneiss (hornblende-biotite gneiss) were formed. Some quartz monzonite gneiss in the large bodies in the northwestern and southeastern parts of the district became mobile after transformation and invaded the surrounding rocks. Relations are clearest near the northwest corner of the district where a funnel-shaped mass more than a mile in diameter forcibly injected and greatly deformed the adjacent rocks. Several fine-grained dacite porphyry dikes, definitely older than the granitic stock, cut the gneiss complex in the vicinity of lower Camp Creek. The dikes follow tension fissures that do not conform to the structural pattern shown by the gneiss complex, and the dacite porphyry is believed to be unrelated to the other pre-Cambrian rocks in the district. An intrusive granitic rock forms a stock and associated dikes in the central part of the Northgate district and several related dikes near the east edge of the district. Similar rocks are common in the Rocky Mountains of southern Wyoming and northern Colorado; and the stock is believed to be a cupola on a much larger underlying body. The original magma, which apparently was dioritic or quartz dioritic in composition, made way for itself by magmatic stoping. After solidification, the dioritic rock was pervasively deformed on a minor scale and invaded by alkaliand silica-bearing magmatic solutions which converted the main body of the stock into a biotite-quartz monzonite. Much of the biotite in the peripheral parts of the stock is chloritized, and the associated plagioclase is significantly more sodic than that in the central part of the stock. Wall-rock alteration

Metamorphism and the origin of granitic rocks, Northgate district, Colorado