Granite Origin Evidence Analysis

This question asks to identify the geological features that strongly suggest granites formed from molten magma (magmatic origin).

Examining Evidences for Magmatic Origin

• Chilled Margins (A): The presence of fine-grained chilled margins around granite bodies indicates that hot magma intruded into cooler surrounding rocks, causing the magma at the contact to cool and solidify quickly. This is a typical sign of magmatic intrusion.
• Perfect Gradation (B): A perfect, gradual transition between granite and the country rock usually points towards metamorphic processes, like granitization, where the rock gradually changes composition in place, rather than magma physically intruding.
• Xenoliths (C): Xenoliths are fragments of the pre-existing country rock that have been broken off and incorporated into the moving magma. Their presence within the granite is a direct piece of evidence that the granite existed as a mobile, molten mass capable of engulfing rock fragments.
• Graphic Texture (D): This texture is characterized by the regular, intergrowth pattern of quartz and feldspar crystals. It is commonly observed in felsic igneous rocks and is thought to form during the crystallization of a granite magma.

Conclusion on Granite Formation Evidence

Based on the analysis, the features providing evidence for the magmatic origin of granites are:

• A. Chilled margins - Shows rapid cooling upon intrusion.
• C. Presence of xenoliths - Indicates magma assimilation.
• D. Graphic texture - A common crystallization texture in magmas.

Feature B (Perfect gradation) is generally associated with metamorphic or metasomatic origins rather than magmatic ones.

Therefore, the correct combination is A, C, and D.