Major and trace element analyses of ODP Leg 120 volcanic rocks (Table 1)

Widespread Lower Cretaceous magmatism occurred along the Indian-Australian/Antarctic margins, and in the juvenile Indian Ocean, during the rifting of eastern Gondwana. The formation of this magmatic province probably began around 120-130 Ma with the eruption of basalts on the Naturaliste Plateau and at Bunbury, western Australia. On the northeast margin of India, activity began around 117 Ma with the Rajmahal continental basalts and associated lamprophyre intrusions. The formation of the Kerguelen Plateau in the Indian Ocean began no later than 114 Ma. Ultramafic lamprophyres (alnoites) were emplaced in the Prince Charles Mountains near the Antarctic continental margin at ~ 110 Ma. These events are considered to be related to a major mantle plume, the remnant of which is situated beneath the region of Kerguelen and Heard islands at the present day.
Geochemical data are presented for each of these volcanic suites and are indicative of complex interactions between asthenosphere-derived magmas and the continental lithosphere. Kerguelen Plateau basalts have Sr and Nd isotopic compositions lying outside the field for Indian Ocean mid-ocean ridge basalts (MORB) but, with the exception of Site 738 at the southern end of the plateau, within the range of more recent hotspot basalts from Kerguelen and Heard Islands. However, a number of the plateau tholeiites are characterized by lower 206Pb/204Pb ratios than are basalts from Kerguelen Island, and many also have anomalously high La/Nb ratios. These features suggest that the source of the Kerguelen Plateau basalts suffered contamination by components derived from the Gondwana continental lithosphere. An extreme expression of this lithospheric signature is shown by a tholeiite from Site 738, suggesting that the southernmost part of the Kerguelen Plateau may be underlain by continental crust.
The Rajmahal tholeiites mostly fall into two distinct geochemical groups. Some Group I tholeiites have Sr and Nd isotopic compositions and incompatible element abundances, similar to Kerguelen Plateau tholeiites from Sites 749 and 750, indicating that the Kerguelen-Heard mantle plume may have directly furnished Rajmahal volcanism. However, their elevated 207Pb/204Pb ratios indicate that these magmas did not totally escape contamination by continental lithosphere. In contrast to the Group I tholeiites, significant contamination is suggested for Group II Rajmahal tholeiites, on the basis of incompatible element abundances and isotopic compositions.
The Naturaliste Plateau and the Bunbury Basalt samples show varying degrees of enrichment in incompatible elements over normal MORB. The Naturaliste Plateau samples (and Bunbury Basalt) have high La/Nb ratios, a feature not inconsistent with the notion that the plateau may consist of stretched continental lithosphere, near the ocean-continent divide.

东冈瓦纳大陆裂解时期，印度-澳大利亚/南极大陆边缘及新生印度洋范围内广泛发育下白垩纪岩浆作用（Lower Cretaceous magmatism）。该岩浆省的形成可能始于约120~130 Ma，以澳大利亚西部纳特拉斯蒂高原（Naturaliste Plateau）和班伯里（Bunbury）地区的玄武岩喷发为标志。在印度东北缘，岩浆活动始于约117 Ma，伴随拉杰马哈勒（Rajmahal）大陆玄武岩及伴生煌斑岩侵入体（lamprophyre intrusions）的形成。印度洋内凯尔盖朗高原（Kerguelen Plateau）的形成时间不晚于114 Ma。约110 Ma时，超镁铁质煌斑岩（Ultramafic lamprophyres, alnoites）侵位至南极大陆边缘附近的查尔斯王子山脉（Prince Charles Mountains）。上述地质事件被认为与一处大型地幔柱（mantle plume）相关，现今该地幔柱的残体位于凯尔盖朗与赫德岛（Heard Islands）下方区域。

本文提供了各火山岩套的地球化学数据，这些数据指示软流圈（asthenosphere）起源的岩浆与大陆岩石圈（continental lithosphere）之间存在复杂的相互作用。凯尔盖朗高原玄武岩的锶（Sr）与钕（Nd）同位素组成超出印度洋洋中脊玄武岩（mid-ocean ridge basalts, MORB）的分布范围，但除高原南端738站位外，其同位素组成落在凯尔盖朗与赫德岛近期热点玄武岩的范围内。然而，部分高原拉斑玄武岩（tholeiites）的206Pb/204Pb比值低于凯尔盖朗岛玄武岩，且多数样品的La/Nb比值异常偏高。这些特征表明，凯尔盖朗高原玄武岩的源区受到了冈瓦纳大陆岩石圈衍生组分的混染。738站位的拉斑玄武岩极端体现了这种岩石圈地球化学特征，暗示凯尔盖朗高原最南端区域下方可能存在大陆地壳。

拉杰马哈勒拉斑玄武岩大致可分为两个地球化学特征迥异的群组。部分I组拉斑玄武岩的Sr、Nd同位素组成及不相容元素丰度（incompatible element abundances）与749、750站位的凯尔盖朗高原拉斑玄武岩相似，表明凯尔盖朗-赫德地幔柱可能直接为拉杰马哈勒火山作用提供了物源。但这些样品较高的207Pb/204Pb比值说明，其岩浆并未完全逃脱大陆岩石圈的混染。与I组拉斑玄武岩不同，基于不相容元素丰度与同位素组成特征，II组拉杰马哈勒拉斑玄武岩被认为经历了显著的岩石圈混染。

纳特拉斯蒂高原与班伯里玄武岩样品相较于正常洋中脊玄武岩，在不相容元素富集程度上存在差异。纳特拉斯蒂高原样品（及班伯里玄武岩）具有较高的La/Nb比值，这一特征与"该高原可能由位于洋陆分界附近的拉伸大陆岩石圈构成"的观点并不矛盾。