According to my project that has been given, how can we expect to have igneous rock with Coarse-grained in top of the intrusive Basalt with fine-grained without metamorphic effect? we have lime stone with fossil in downer part without metamorphic effect too.
How can it be possible?what is the exact Interpretation in first step?
Dear Mr. Rahimpour,
given an almost equal chemical composition throughout the series of volcanic rocks, changes in the texture from fine to coarse can result from different processes without any metamorphic process impacting on the rock series. There are cooling phenomena, e.g. when getting in contact with an aquatic system on land or on the sea bottom. Fluid escape structures may also come into existence. As long as you discover fossils in the associated sediments and you are still able to denominate your calcareous rocks as limestones, any regional metamorphism you can exclude in this setting. Local changes at the interface between basalt and its wall rocks are, however, possible. Such contact metamorphic processes leading to rather uncommon minerals like mullite in pelites or spurrite in carbonate rocks might be expected. Without further information on the overall litholog any attempt of an in-depth explanation is, however, futile.
With kind regards
H.G.Dill
Some stray thoughts!
The coarse part might be accumulation of phenocrysts due to settling at the bottom of the sill. But that would require overturned folding after intrusion of the dyke.
Flowage differentiation?
Volatile transfer to upper parts in thick intrusions.
By the way, many years ago, AR McBirney explained some layering in the Skaergaard intrusion to have resulted from diffusion.
Best,
Qasim Jan
it is good explanation from Harald G. Dill
my explanation will consider the cooling processes, it will be thermodynamic factor. I think it is not intrusive as it is recall for side effect.
it happened same as gabbroid rocks in same environment as layered gabbro.
in down you will see dolerite ( microgabbro) , norite , and troctolite , and in top very coarse anorthosite. it is interaction between chemical and physical processes
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