Emplacement Age and Crustal Assimilation of Rift‐Related Mafic Magmatism in the Tiger Gabbro Complex, Northern Victoria Land, Antarctica

Mafic igneous rocks with mixed geochemical characteristics and equivocal emplacement ages (~535 Ma vs. ~497 Ma) have complicated the interpretations of the petrogenetic and tectonic evolution in the Tiger Gabbro Complex (TGC), northern Victoria Land, Antarctica. To solve this issue, we carried out geochemical analyses including bulk‐rock as well as zircon U–Pb, O, and Hf isotopic compositions of gabbroic rocks that are conspicuous in two lithologic units: (1) layered gabbros intruding upper meta‐sedimentary rocks and (2) porphyritic gabbros/hornblendites in the lower section. The former belongs to the calc‐alkaline to tholeiitic series with normal mid‐ocean ridge basalt affinities, whereas the latter belongs to the tholeiitic series with back‐arc basin basalt‐like affinities. Bulk‐rock Sr–Nd isotopic compositions of layered vs. porphyritic gabbros yielded 87 Sr/ 86 Sr (t) = 0.7039–0.7048 and 0.7029–0.7034, and εNd (t) = +1.92–3.47 and + 4.44–5.10, respectively, resulting from ~5–15% crustal contamination. The assimilation is corroborated by the presence of zircon xenocrysts (526–2,873 Ma) from the former with δ 18 O = 4.5– 8.0‰ and εHf (t) = – 32.2 to + 2.2. In contrast, zircon autocrysts from the latter yielded the weighted mean 206 Pb/ 238 U age of 495 ± 11 Ma ( t σ) with mantle‐like δ 18 O = 5.0–5.5‰ and εHf (t) = + 8.5–20.5. Taken together, the TGC represents a mafic magma chamber in an intra‐arc rift synchronously formed with the back‐arc basin at ~495 Ma.