Coucal formation

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The Coucal formation is of predominantly acidic volcanic existing formation from the paleoarchean Western Australia . It forms part of the East Pilbara Terrane and is the second formation from the Coonterunah subgroup belonging to the Warrawoona Group . It was dated 3515 to 3498 million years BP and thus comes from the Isuum .

etymology

The pheasant spur cuckoo Centropus phasianinus found in the outcrop area

The Coucal formation was named after the spur cuckoos , which are known as Coucal in English .

Occurrence

The occurrence of Coucal lineup is on the south of the Carlindi-granite complex located Pilgangoora greenstone belt limited in the other greenstone belts of Eastern Pilbara Craton missing.

introduction

The Coucal Formation, which is up to 1,500 meters thick on average, is of magmatic origin and predominantly extrusive in nature. It consists of rhyolites , rhyodacites and acidic tuffs , but there are also andesites , dazites and, above all, thloeiitic basalts . These rocks were later silicified, recrystallized and metamorphosed to varying degrees (green slate to amphibolite facies). The Coucal formation concordantly overlays the table top formation with a basal chert layer . It is in turn overlaid by the double bar formation .

stratigraphy

Tholeiitic basalts follow above the Grenzchert . For the first time, up to 35 meters of intermediate to acidic volcanoclastics were deposited above this between two other delimiting chert layers. These are covered by another tholeiitic basalt series, up to 1000 meters thick, into which two intrusive layers with gabbros have penetrated. The characteristic acidic volcanic rocks only appear in the hanging wall of the formation. The formation ends with a double chert layer, which separates it from the concordant double bar formation.

Lithology

The Metachertlagen are up to 8 meters thick. They consist of alternating light and dark layers in the millimeter to centimeter range. The dark layers are rich in magnetite or organic carbon (kerogen). The overlying volcanoclastics consist of bimodal tephra . They also contain micritic carbonate layers with varying degrees of silicification. A variant are red layers enriched in hematite , which are replaced with layers of white, pure, cryptocrystalline quartz . The tholeiitic basalts are fine-grained and show occasional cushions , in general they are very similar to comparable units of the underlying table-top formation. Sub-volcanic intrusives such as dolerites and gabbros occur in them. The intermediate to acidic volcanic rocks in the hanging wall are typically brecciated and weather to round shapes. These include very vesicle-rich, hyaloclastic breccias with a Dazite to rhyolite composition as well as flow-banded Dazite. Individual flow units can be up to more than 200 meters thick. The autotraction of these volcanic rocks is characteristic of the deposition of acidic rock sequences in deep water.

mineralogy

The acidic volcanic rocks are made up of phenocrystals , matrix and amygdalen (almonds). The phenocrystals are 1 to 5 millimeter quartz and rectangular feldspars , which are very strongly converted to pyrophyllite . The highly silicified and sericized base mass is beige to light green in color. Their crystals are very fine-grained (<0.1 millimeter) quartz and muscovites that convert to albite , small amounts of chlorite and traces of opaque minerals (oxides). The approximately 1.5 cm large amygdals are stretched out.

Petrology

In the Coucal formation of the Coonterunah subgroup, two acidic, sodium- emphasized magma series (K 2 O / Na 2 O = 0.050-0.045) can be distinguished:

  • Members of the CF-1 series have lower Ti and Fe contents and their trace element composition is more fractionated . A low Yb content and a high La / Yb ratio indicate that there is only minor contamination from TTG magma components.
  • The CF-2 series is much more typical of an origin from fractionated tholeiites (and not from calcareous or archaic TTG magma series), recognizable by a strong Fe emphasis, low K 2 O content (<1.0 percent by weight) and increased concentrations heavy rare earths (HREE) and Y , which in turn are positively correlated with the SiO 2 content and the La / Yb ratio.

A Dazit that very likely emerged from a TTG magma is outside of these two series. It is characterized by very low Yb contents and a very low La / Yb ratio.

Dating

In 1995, Buick and colleagues were able to use the SHRIMP (uranium-lead dating in zirconia ) method to date dazites and rhyolites from the upper Coucal formation to 3515 ± 3 million years BP. For a dacite, Green (2001) found 3518 ± 4 million years BP. The Coucal formation thus belongs to the outgoing isuum .

Individual evidence

  1. ↑ a b R. Buick et al: Record of emergent continental crust ~ 3.5 billion years ago in the Pilbara Craton of Australia . In: Nature . tape 375 , no. 6532 , 1995, pp. 574-577 , doi : 10.1038 / 375574a0 .
  2. ↑ a b c M. G. Green: Early Archaean crustal evolution: evidence from ~ 3.5 billion year old greenstone successions in the Pilgangoora Belt, Pilbara Craton, Australia . Sydney 2001 (PhD thesis, University of Sydney).
  3. ↑ Fisher, RV and Schmincke, H.-U .: Pyroclastic rocks . Springer Verlag, Berlin; New York 1984.
  4. ^ Van Kranendonk, MJ et al .: Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia . In: Terra Nova . tape 19 , 2007, pp. 1-38 , doi : 10.1111 / j.1365-3121.2006.00723.x .