Table top formation
The table-top formation from the Paleoarchean ( Isuum ) is the oldest supracrustal formation from the Pilbara supergroup of the Pilbara craton in Western Australia . In the East Pilbara Terrane, it forms the basis of the Coonterunah subgroup belonging to the Warrawoona Group .
Occurrence
The occurrence of the table-top formation is limited to the Pilgangoora (formerly also East Strelley Greenstone Belt ) and the Pincunah greenstone belt in the northwest of the East Pilbara Terrane ; it is absent in the other greenstone belts .
introduction
The approximately 2000 to 2500 meters thick table top formation is of magmatic origin and predominantly extrusive in nature. It consists mainly of fine-grain basalts ( grain size around 0.5 millimeters). These rocks were later silicified, recrystallized and metamorphosed to varying degrees (green slate to amphibolite facies). The original base of the formation is nowhere exposed, but there are intrusive and tectonic contacts to the underlying Carlindi granite complex . The table-top formation is concordantly overlaid by the Coucal formation , which begins with a distinctive chert layer .
stratigraphy
In the horizontal of the table-top formation, about 250 meters of magnesium- rich basalts follow over tholeiitic basalts, the only ones of their kind in the entire Coonterunah subgroup. These are variolithic basalt rivers with beautifully developed varioles made of albite and actinolite (up to 50 millimeters in diameter). About 20 meters of pyroxene- bearing, komatiitic basalts lay above this , the pyroxene needles of which have now been completely converted to amphibole . The rest of the sequence is dominated by generally massive, very low-bubble tholeiite basalts, which only form pillow lava in places . The rare pillows, measuring 50 to 150 centimeters in length, are well preserved and show the typical structure of the core, edge zone and skin. A special feature are the obliquely layered ripples from volcanic sediments, which can be found on the scoria-containing upper side of some more bubble-rich basalt rivers.
The intrusive rocks penetrated into the sequence show a structural continuum from sub-volcanic dolerites to gabbros . These mafic intrusive panels are more or less concordant and usually end in the hanging wall with a basalt river. Cumulus xenolites of these mafic intrusives occur in the lower table-top formation.
Deposition environment
The table-top formation was deposited under the sea, mainly in the deep area. The rare ripples indicate a flattening of the sedimentation environment.
Petrology
The predominant, mafic tholeiite basalts can generally be divided into two magma sequences, one with a high titanium content (> 0.8 percent by weight TiO 2 ) and one with a low Ti content (<0.8 percent by weight TiO 2 ). The low Ti basalts come from a more depleted mantle region than the high Ti basalts. Their trace element pattern makes them recognizable for the rare earths as only moderately fractionated N-MORB basalts.
The only weak, ultra-mafic komatiite rivers are not depleted in aluminum and have an MgO content of 22 to 30 percent by weight.
metamorphosis
Due to the penetration of the Carlindi granite complex, volcanic rocks and intrusiva of the table-top formation were metamorphically changed under pressures of 0.2 to 0.35 GPa . Typical minerals of the green stone association emerged. As a result, the mineral inventory is now composed of amphibole (25 to 50% ferro-actinolite , in the amphibolite facies aluminum-containing ferrotschermakite ), albite (25 to 50%), epidote (10%), chlorite (0 to 20%) and quartz (5 up to 20%). Clinozoisite , leukoxene and the oxides magnetite and ilmenite are used as accessories, and margarite (up to 10%) and carbonates are occasionally added. Nevertheless, tectonically less stressed sections in coarse-grained rocks give an idea of the original igneous minerals clinopyroxene , orthopyroxene , plagioclase and hornblende in the form of exsolution lamellae. Rocks that are particularly rich in bubbles show signs of transformation to anchorite .
Age
The age of the table-top formation is still pending. However, the overlying Coucal Formation has been dated to 3515 ± 3 million years BP , which justifies a minimum age of around 3517 million years BP for the table-top formation. The intrusion of the Carlindi Granite Complex occurred around 3484 to 3468 million years BP.
Individual evidence
- ↑ Green, MG: Early Archaean crustal evolution: evidence from ~ 3.5 billion year old greenstone successions in the Pilgangoora Belt, Pilbara Craton, Australia (doctoral thesis) . University of Sidney 2001.
- ↑ Harnmeijer, J .: Squeezing blood from a stone: inferences into the life and depositional environments of the Early Archaean (doctoral thesis) . University of Washington 2010.
- ↑ Smithies, RH et al .: It started with a plume - early Archaean basaltic proto-continental crust . In: Earth and Planetary Science Letters . tape 238 , 2005, pp. 284-297 .
- ↑ a b Buick, R. et al .: Record of emergent continental crust ~ 3.5 billion years ago in the Pilbara Craton of Australia . In: Nature . tape 375 , 1995, pp. 574-577 .