Final answer:
Garnet and staurolite indicate a high grade metamorphic rock because these minerals form under high temperatures and pressures typical of intense metamorphic conditions, like those in terranes with significant tectonic activity.
Step-by-step explanation:
The mineral combinations that would best indicate a high grade metamorphic rock are Garnet and staurolite. This is because, during the process of metamorphism (the transformation of existing rocks under the influence of heat and pressure), new minerals can form in the rock, reflecting the conditions of heat and pressure to which the rock was subjected. Minerals such as garnet and staurolite are known to form under high temperatures and pressures, and seeing these minerals, particularly as porphyroblasts (larger crystals within a finer-grained matrix), is a strong indication of high-grade metamorphism. In contrast, minerals like quartz, feldspar, calcite, dolomite, halite, and gypsum can form in a variety of geological settings, including lower grade metamorphic conditions.
For example, gneiss is a type of high-grade metamorphic rock that often has bands or stripes of light and dark minerals due to intense metamorphism, and minerals like garnet and staurolite could be present in its matrix. The presence of certain index minerals can be indicative of the metamorphic grade, and thus, the identification of garnet and staurolite specifically points to higher grades of metamorphism, typically associated with conditions found in terranes where there has been significant tectonic activity, such as continent-continent collisions.