A mantle plume connection for alkaline lamprophyres (sannaites) from the Permian Tarim Large Igneous Province: Petrological, geochemical and isotopic constraints
A mantle plume connection for alkaline lamprophyres (sannaites) from the Permian Tarim Large Igneous Province: Petrological, geochemical and isotopic constraints
复制标题
二叠纪塔里木大火成岩省碱性煌斑岩（sannaites）的地幔柱连接：岩石学、地球化学和同位素约束
DOI：
10.1093/petrology/egad004
复制
发表时间：
2023-01
影响因子：
3.9
通讯作者：
Lukáš Krmíček
中科院分区：
文献类型：
--
作者：
Bingxiang Liu;Zhaochong Zhang;Andrea Giuliani;Qiuhong Xie;Weiliang Kong;Changhong Wang;Bowen Wei;Shan Ke;M. Santosh;Bo Zhang;Xingchao Zhang;Lukáš Krmíček
与大火成岩省（LIPs）相关的煌斑岩的起源仍然存在争议，特别是它们是由地幔柱的直接熔融产生的，还是来自热扰动次大陆岩石圈中先前的交代域。在这里，我们报告了最近发现的一套碱性煌斑岩（三色岩）的岩石学和地球化学特征，它们代表了中国西北部塔里木二叠系大火成岩省岩浆活动的最后脉冲。在斜长石、斜辉石、霞石、角闪石、黑云母和钛磁铁矿的地质体中，三晶石呈斑状结构，由橄榄石、斜辉石、角闪石、绿云母和钛磁铁矿组成，并伴有少量黄铁矿和磷灰石。基岩中的碳酸盐石英和几乎纯净的钠长石分别由晚期岩浆分离产生的不混溶碳酸盐和含水流体结晶而成。岩石中SiO2 （37.7 ~ 49.3 wt.%）和MgO （2.74 ~ 9.91 wt.%）含量低至中等，Fe2O3T （22.7 wt.%）和碱（Na2O+K2O）含量高（9.02 wt.%）。它们具有高不相容元素丰度的特点，特别是大离子亲石元素（Rb和Ba）和轻稀土元素（La和Ce）相对于P和高场强元素（P和Ti）明显富集。它们的δ66Zn值在0.22 ~ 0.46‰范围内，平均为0.37±0.04‰（2SE, n=10），明显大于morb的δ66Zn值（0.27±0.05‰）。它们的（87Sr/86Sr）t值在0.7035 ~ 0.7061之间，εNd(t)值在-0.97 ~ +5.62之间，δ26Mg值在-0.36‰~ -0.17‰之间（n=8），后者与全球morb一致。根据锌同位素与TiO2-FeO浓度的相关性，我们推断部分三绿岩中的重锌同位素是由Fe-Ti氧化物矿物的分离结晶引起的。这些岩石的全岩石地球化学特征（负K异常和大离子亲石元素富集）和流纹岩-熔体模拟表明，三绿岩的原生岩浆来源于含角闪岩的富岩石圈地幔。含角闪岩的交代地幔的交代作用和相关形成可能与地幔柱活动早期源自塔里木地幔柱的岩石圈下熔体/流体有关，而不是像以往对其他碱性幔源岩浆的研究那样认为是源自板块的流体或碳酸盐熔体。在这一阶段，塔里木地幔柱的热输入可能引发了部分熔融。研究表明，在岩石圈地幔交代作用的复杂旋回和先前富集域的后期熔融过程中，大型地幔柱的多阶段演化过程中可能产生外来的富碱岩浆。
The origin of lamprophyres associated with Large Igneous Provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed sub-continental lithosphere. Here we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim Large Igneous Province in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O+K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g., La and Ce) relative to P and high-field-strength elements (e.g., P and Ti). They show a relatively restricted range of δ66Zn values between 0.22 and 0.46 ‰ with an average of 0.37 ± 0.04‰ (2SE, n=10), which is significantly heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from –0.97 to +5.62, and δ26Mg from –0.36‰ to –0.17 ‰ (n=8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2-FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe-Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and Rhyolite-MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle. Metasomatism and related formation of amphibole-bearing metasomatised mantle may be linked to sub-lithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during this stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains.