Zircon ages and Hf isotopic compositions of plutonic rocks from the Central Tianshan (Xinjiang, northwest China) and their significance for early to mid-Palaeozoic crustal evolution

<div><p>We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475–473 Ma) granitoid rocks have zircon <i>ε_Hf</i>₍<i>_t</i>₎ values either positive (+0.3 to +9.5) or negative (−6.0 to −12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician–Silurian (ca. 458–425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458–442 Ma) has negative zircon <i>ε_Hf</i>₍<i>_t</i>₎ values (−6.3 to −13.1), indicating a predominantly crustal source; zircon from later events (ca. 434–425 Ma) has positive zircon <i>ε_Hf</i>₍<i>_t</i>₎ values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410–404 Ma) rocks have near-zero zircon <i>ε_Hf</i>₍<i>_t</i>₎ values, either slightly negative or positive (−1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (−11.2 to −14.8). The Late Devonian (ca. 368–361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock <i>ε_Nd</i>₍<i>_t</i>₎values (−2.4 to −5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle–crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust.</p></div

Nd–Sr isotopic constraint to the formation of metatexite and diatexite migmatites, Higo metamorphic terrane, central Kyushu, Japan

<div><p>ABSTRACT</p><p>Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite-facies zones of the Higo low-<i>P</i>/high-<i>T</i> metamorphic terrane. Here we report Nd–Sr isotopic and whole rock composition data from an outcrop in the highest-grade part of the granulite-facies zone, in which diatexite occurs as a 3 m-thick layer between 2 m-thick layers of stromatic-structured metatexite within pelitic gneiss. The metatexite has Nd–Sr isotopes and whole rock compositions similar to those of the gneiss, but the diatexite shows the reverse. The diatexite has a higher ε_Nd(t) and ¹⁴⁷Sm/¹⁴⁴Nd ratio (ε_Nd(t) = −0.5; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1636) than the gneiss (ε_Nd(t) = −2.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1287) and metatexite (ε_Nd(t) = −3.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1188). The (⁸⁷Sr/⁸⁶Sr)_initial and ⁸⁷Rb/⁸⁶Sr of the diatexite ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70568; ⁸⁷Rb/⁸⁶Sr = 0.416) are lower than those of the gneiss ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70857; ⁸⁷Rb/⁸⁶Sr = 1.13) and metatexite ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70792; ⁸⁷Rb/⁸⁶Sr = 1.11). The metatexite and gneiss show enrichment of Th and depletion of P and Eu and have a similar chondrite-normalized REE pattern, which shows steep LREE–MREE-enriched and gently declining HREE patterns and negative Eu anomalies, whereas the diatexite shows enrichment of Sr and depletion of Th and Y, and exhibits gently declining LREE and steeply declining HREE pattern and weak Eu depletion. The metatexite migmatite is interpreted to have formed by <i>in situ</i> partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived melt with a juvenile component. The Cretaceous high-temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle-derived basalts under a volcanic arc along the eastern margin of the Eurasian continent, and mass transfer and advection of heat <i>via</i> hybrid silicic melts from the lower crust.</p></div