The question asks for explanations for negative Europium (Eu) anomalies observed in upper crustal rocks such as granite and granodiorite. A negative Eu anomaly on a chondrite-normalized rare earth element (REE) diagram indicates that the rock contains less Eu than expected relative to its neighboring REEs (Samarium, Sm, and Gadolinium, Gd).

Understanding Eu Anomalies in Granite

Negative Eu anomalies in felsic rocks like granite and granodiorite typically arise from processes occurring during magma evolution, specifically fractional crystallization.

Reasoning for Correct Options

• Option A: These rocks are end-products of magmatic differentiation
  Granites and granodiorites are often formed late in the sequence of magmatic differentiation. Fractional crystallization removes certain elements and minerals, leading to changes in the composition of the remaining melt. Eu behaves differently from other lanthanides because it can exist as Eu²⁺ (similar in size to Ca²⁺) in addition to Eu³⁺. Plagioclase feldspar, a common mineral in felsic magmas, preferentially incorporates Eu²⁺. As plagioclase crystallizes and is removed from the magma, the remaining melt becomes depleted in Eu, resulting in a negative Eu anomaly in the rocks (like granite) that crystallize from this evolved melt. This explanation aligns with the concept of magmatic differentiation.
• Option D: The melt residues contain plagioclase which are enriched in Eu
  This statement refers to the minerals that crystallize out of the magma (melt residues in the context of fractionation). Plagioclase feldspar strongly prefers to incorporate Eu²⁺ during crystallization compared to other REEs. When plagioclase crystallizes and is removed from the magma, it takes Eu with it. Consequently, the remaining liquid (which eventually forms the granite or granodiorite) becomes relatively depleted in Eu, leading to the observed negative Eu anomaly.

Explanation for Incorrect Options

• Option B: These rocks were formed by melting of the mantle, which was already depleted in Eu
  Upper crustal rocks like granite are typically formed from crustal melting or extensive fractional crystallization of mantle-derived magmas, not directly from melting of a Eu-depleted mantle source in a way that explains negative anomalies in the final product. Mantle melting usually produces more primitive compositions.
• Option C: Most of the Eu was incorporated in other minerals
  While true in principle, Option D is more specific and accurate by identifying plagioclase as the key mineral responsible for incorporating Eu, and clarifying that this incorporation happens in the removed fractions (residues).

Therefore, the formation of granite and granodiorite as end-products of magmatic differentiation (A) and the preferential incorporation of Eu into crystallizing plagioclase (D) are the primary reasons for negative Eu anomalies in these rocks.