ABSTRACT

The Dutsen Wai and Ropp complexes are parts of the over 50 within-plate alkaline ring complexes in north-central Nigeria. These complexes consist mainly of A-type granites that were emplaced between 175 Ma (in Dutsen Wai) and 149 Ma (in Ropp). They are characterized by moderately negative zircon ԑ_Hf(t) values (–9.8 to – 5.86) with Paleoproterozoic (1597–1884 Ma) crustal model ages (T_DMc), suggesting the mantle-derived parental magmas have been substantially modified by assimilation of crustal materials. The granites exhibit elevated HFSEs (e.g., Nb and Zr) and LILE (e.g., Rb and REE) concentrations, with higher values in the peralkaline than in the aluminous granites, but are depleted in compatible elements (e.g., CaO, MgO, Ba and Sr). This is typical of highly differentiated granites associated with rare metal mineralization. The peralkaline plutons show REE patterns with strong negative Eu anomalies, significant M-type lanthanide tetrad effect (TE_1,3 = 1.16–1.32), and non-CHarge and Radius Controlled (CHARAC) behaviour of some HFSEs. The positive correlation of Na₂O vs. Nb (r² = 0.92) and Nb vs. TE_1,3 (r² = 0.7), increasing degree of albitization and shift in eutectics of granite minimum towards Ab-Qtz sideline indicate that Na-rich fluid plays an important role in the complexation between F and other incompatible elements in open system processes. Fluorine also favoured the retention of these crucial metals in the peralkaline melt until late magmatic-stage during which the breakdown of the F-HFSEs and F-REE complexes was accompanied by crystallization of pyrochlore and REE-bearing accessory minerals. We proposed that the emplacement of these A-type granites was probably linked to the prevailing transtension regime before the fragmentation of Gondwana during the Mesozoic period which reactivated some deep-seated Pan-African transcurrent faults. This, in turn, caused pressure release and channelled fluid, which triggered partial melting of the lower part of the lithospheric mantle.

摘要

Dutsen Wai 和 Ropp 复合体是尼日利亚中北部 50 多个板内碱性环复合体的一部分。这些复合体主要由位于 175 Ma（在 Dutsen Wai）和 149 Ma（在 Ropp）之间的 A 型花岗岩组成。它们的特征是中度负的锆石 ⑩ _Hf (t) 值（–9.8 至 – 5.86），古元古代（1597-1884 Ma）地壳模型年龄（T _DMc)，表明地幔来源的母岩浆已被地壳物质的同化作用显着改变。花岗岩表现出较高的 HFSE（例如，Nb 和 Zr）和 LILE（例如，Rb 和 REE）浓度，在过碱性中的值高于铝质花岗岩，但相容元素（例如，CaO、MgO、Ba 和先生）。这是与稀有金属矿化相关的高度分化花岗岩的典型特征。所述过碱性岩体显示具有强的负Eu异常，显著M型镧系元素四重效应（TE REE模式_1,3  = 1.16-1.32），和非CH ARGE和- [R adius Ç ontrolled （CHARAC）一些高场强元素的行为。Na ₂正相关O vs. Nb ( r ²  = 0.92) 和 Nb vs. TE _1,3 ( r ² = 0.7)，花岗岩的共晶最小值向 Ab-Qtz 侧线移动的增加程度和偏移表明富钠流体在 F 与开放系统过程中其他不相容元素之间的络合中起着重要作用。氟也有利于将这些关键金属保留在高碱性熔体中，直到岩浆晚期，在此期间 F-HFSE 和 F-REE 配合物的分解伴随着烧绿石和含 REE 副矿物的结晶。我们提出，这些 A 型花岗岩的就位可能与中生代冈瓦纳纪破碎之前盛行的横张机制有关，这使一些深部泛非横流断层重新活动。这反过来又导致压力释放和引导流体，