Dear Mr. Alshaghdari,

I would like to refer to good papers dealing with ophiolites and textbooks where the processes leading to massive and layered gabbros are extensively treated.

With kind regards

Harald G. Dill

Hi Mohsen, layered gabbros in ophiolites are commonly interpreted in terms of fractional crystallization beneath oceanic spreading centers in magma chambers. Cheers Daniel

Because they cool slowly. Their minerals are relatively dense and heavy and cool in deep parts of the crust/mantle where temperature is warmer than the areas where felsic intrusions cool down and crystallize.

thanks all. if there is more interpretation will be better

Layered gabbroids are differentiated intrusions of open and long-lived intermediate chambers through which large volumes of magma passed. Their differentiation is explained by the fact that the velocity of magma in the chamber drops by 5-6 orders of magnitude. The model of dynamic differentiation and crystallization consists of three main axioms and two additional ones (Radko, 1991). 1. The chamber localizing the differentiated intrusion is an intermediate focus that has supply and output channels, that is, it is a hypabyssal subvolcano. 2. Magma passing through the chamber resets part of the solid phase in it. 3. The melt enters the chamber many times, forming various facies.

Dear Mohsen: all I can add to Ivan's answer is that crystal fractioning in large basaltic chambers, either as continental lopoliths or as the lowest part of the ophiolitic suite or as the lowest part of submarine plateaus, is very effective mainly due to the low viscosity of the mafic magma, which allows dense crystals to settle in the bottom of the chambers, forming the so called cumulates or cumulative rocks. In all Igneous Petrology books you will find a concise description of this process of magma differentiation. Cumulative textures are well explained in the old text of Ehlers & Blatt "Igneous, Sedimentary and Metamorphic Petrology". But the seminal paper in this subject is the famous Wager & Brown works on the well known Skaergaard Stratiform Complex in SE Greenland, which references you can find in any Petrology book and at the end of the Namur et al. paper. Two interesting and remarkable new papers on this fascinating subject are:

Research Gate, 2015:

Igneous layering in basaltic magma chambers

Namur, O; Benedicte, A.; Boudreau, A.; Bush, J.W.M.; et al.

Research Gate, 2005

Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from Stillwater Complex

Spandler, C.; Arculus, R. & Mavrogenes, J.A.

Regards, Sebastián.

Fractional crystallisation + crystal settling (in a manner similar to Stoke's particle settling). I co-authored a paper dealing with the mechanics of crystal settling and fomartion of cumulates (where cumulates are mono- or poly-mineralic layers formed in magma chambers):

Article Settling and compaction of olivine in basaltic magmas: An ex...

Let me add that other than AFC and crystal setteling the 'double diffusion' is prime rulling process for deciding the 'layered fabric' within the magma chamber. Here I am attaching excellent paper which may help you.

Regards.

Dejar Saurabh: double diffusion is the process that could explain the origin of mineralized stratiform ores such as the Merensky Reef and chromitite or magnetite layers, I don’t think it is responsible of the whole stratiform cumulate sequence. Regates, Sebastián

Respected sir, I am not expert to it and you may be right. However, what I have read in some basic paper on 'layered complexes' suggest that the alternate banding of 'Cumulates' caused due to 'double diffusion' provoked convection within long-lived and relatively stable magma chmaber and if the alternate layering is common in any pluton regardless to their felsic or mafic nature then the probable reason would be 'double diffusion'. Moreover, I need more discussion to clear my concepts. Regards.

Dear Saurabh: there are so many different layers in a stratiform gabroid body that is quite unlikely all of them were caused by double diffusion, if we understand double diffusion as crystallization in the limit between two different batches of magma: one evolved and the other one fresh; then a complete stratiform sequence would need an enormous number of magma recharges, moreover, many structures found in cumulates point to "turbiditic-like" sedimentary processes, such as grading, scour and fill, and crossed stratification. The problems of stratified grabbros are far for been resolved yet, it is a very complicated matter! Regards, Sebastián.

So, Does it mean that 'magma mixing and mingling' is also very crucial to form 'layered structure'? Re gards.

all

thanks , what exactly double diffusion ? what action performed

Dear Mohen,

You have gotten lot a good answer from bigger experts like me. Perhaps will help the next photos... and if you try to make examination in the proposed direction: On the photos you have tree type rocks (metamorphit, sediment, and layered gabbro… Make a question why the sedimentary, metamorphite rocks are layered… you will get such a response witch can help you… Same you have to examine the minerals in the nearby of layered plan … The mineral types their density in the surface, you have to make a comparison with the rest mineral… You have to make the understanding geologically surrounding of Gabbro body…

Best Regards,

Laszlo

I have forgotten: