Deciphering crustal growth in the southernmost Arabian Shield through zircon U-Pb geochronology, whole rock chemistry and Nd isotopes

New U-Pb zircon geochronology using high-spatial resolution secondary ion mass spectrometry fills a data gap and provides crystallization ages for granitoids from the Asir composite terrane in the southernmost Arabian Shield of Saudi Arabia. Ages of c. 810–685 Ma, c. 663–636 Ma, and 625–610 Ma reflect oceanic island arc genesis, subduction-related arc accretion (syn-collisional), and post-collisional stabilization, respectively. All samples have juvenile ε_Nd(t) compositions with no evidence of older material being involved in their genesis, indicating that this part of the Arabian Shield grew through juvenile magmatic addition and that assimilation by syn- and post-tectonic magmatism involved an isotopically juvenile component(s). The crustal thickness derived from the (La/Yb)_N proxy indicates significant thickening from 10–20 km to c. 70 km at c. 650 Ma, consistent with timing of orogenic uplift and increasing crustal thickness post-dating peak Nabitah orogeny. The age of an intrusion cross-cutting the Atura formation, when combined with other data, provides a well-constrained depositional age of c. 646–625 Ma for the Atura formation and indicates that erosion of the orogenic edifice in this part of the Arabian Shield began at latest by 625 Ma. Our new data indicate that denudation occurred 80–100 m.y. before the development of the prominent sub-Cambrian peneplain, consistent with previous assertions that major pulses of denudation occurred prior to the waning stages of Nabitah orogenesis.

本研究利用高空间分辨率二次离子质谱(secondary ion mass spectrometry)开展新型U-Pb锆石地质年代学(U-Pb zircon geochronology)研究，填补了相关数据空白，并为沙特阿拉伯最南部阿拉伯盾地(Arabian Shield)的阿西尔复合地体(Asir composite terrane)内的花岗质岩石提供了结晶年龄数据。三组年龄区间分别为约810–685 Ma、约663–636 Ma以及625–610 Ma，依次对应洋岛弧成因、俯冲作用相关的弧增生(arc accretion)（同碰撞阶段）以及碰撞后地壳稳定过程。所有样品均呈现新生型ε_Nd(t)同位素组成，未发现有古老物质参与其成因的证据，表明阿拉伯盾地该区域是通过新生岩浆加积作用形成的，且同构造期与后构造期岩浆作用的同化混染过程涉及同位素亏损的新生端元。基于(La/Yb)_N比值代理指标推算的地壳厚度显示，在约650 Ma时，地壳厚度从10–20 km显著增厚至约70 km，这与纳比塔造山运动(Nabitah orogeny)峰期之后的造山隆升及地壳厚度持续增加的时间节点相吻合。对穿切阿图拉组(Atura formation)的侵入体进行定年的结果，结合其他数据，可将阿图拉组的沉积年龄严格限定为约646–625 Ma，同时表明阿拉伯盾地该区域的造山构造体剥蚀作用最晚始于625 Ma。本研究的新数据显示，地表剥蚀作用发生于标志性亚寒武纪准平原(sub-Cambrian peneplain)形成前的80–100百万年，这与此前提出的“主要剥蚀脉冲事件发生于纳比塔造山运动(Nabitah orogenesis)衰退阶段之前”的论断一致。