Trondhjemite petrogenesis modeling by using RhyoliteMelts

[yanweiyao]

Hi,
Now I am using RhyoliteMelts program to simulate the petrogenesis of plagiogranite, particularly of trondhjemite. However, I never obtained the exact trondhjemitic melts composition (SiO2 > 63% while K2O <1%) and the phases crystallized do not match our field data.

For my modeling, I used basaltic melts (low K, 0.1 to 0.2 wt.%) as starting materials and set up changing temperature (1300â„Æ' to 600â„Æ') but constant pressure (I tried both 2 kb and 0.7 kb) with various water content (0 to 3 wt.%) and oxygen fugacity (QFM -1 to 3).

For the modeling results which should mimic the real data, we predict that we should have quartz as the cumulate phase at relatively late stage (e.g. SiO2 reaches around 60 wt.% and after), since the parent basalt should evolve from diorite, quartz diorite to trondhjemite. However, the problem is we never have any quartz until all the remaining liquid disappeared, which means that for the evolving process, no intermediate composition, such as quartz diorite, is obtained. Also, in our modeling, the K keeps increasing to extremely high value (~6 wt.%) while in plagiogranite, it should be less than 0.7 wt.%.

So we are wondering if there is anything wrong with our modeling? Or can this program be used for simulating the plagiogranite petrogenesis and include plagiogranite data in your database? We will really appreciate if you can give us some suggestions.

Thanks,

Weiyao