Geochemistry of ultrapotassic volcanic rocks in Xiaogulihe NE China: Implications for the role of ancient subducted sediments
Geochemistry of ultrapotassic volcanic rocks in Xiaogulihe NE China: Implications for the role of ancient subducted sediments
复制标题
DOI：
10.1016/j.lithos.2014.08.026
复制
发表时间：
2014-11
期刊：
影响因子：
3.5
通讯作者：
Yang Sun;Jifeng Ying;Xin-hua Zhou;J. Shao;Zhuyin Chu;B. Su
中科院分区：
文献类型：
--
作者：
Yang Sun;Jifeng Ying;Xin-hua Zhou;J. Shao;Zhuyin Chu;B. Su
黑龙江省西部的小古力河地区是我国东部迄今为止所报道的唯一一次超钾质火山岩喷发。这些超钾质岩石的特征是K2 O含量极高（> 7重量%），异常非放射成因铅同位素组成（~（206）Pb/~（204）Pb = 16.44-16.55;~（207）Pb/~（204）Pb = 15.39-15.46;~（208）Pb/~（204）Pb = 36.35-36.61），~（87）Sr/~（86）Sr比值较高（0.7053-0.7057），与美国西北部和阿尔丹地盾广泛分布的超钾质火成岩基本一致。~（187）Os/~（188）Os和~（1/Os）之间的正相关关系表明，这些超钾质岩石在岩浆上升过程中可能经历了可忽略的下陆壳增量（小于1%）。这些超钾质岩石的高K2 O含量和87 Sr/86 Sr与206 Pb/204 Pb的负相关性，表明其地幔源区存在钾质相，主要是金云母。稀土元素分馏作用强，Nd-Hf同位素不解耦，表明含石榴子石烃源岩发生了低程度的部分熔融。此外，全岩成分中CaO和Al 2 O3含量较低，橄榄石斑晶化学成分中Fe/Mn比值较低，表明小古力河超钾质岩的主要烃源岩为橄榄岩而非辉石岩。基于这些独特的地球化学特征，我们认为小古力河超钾质火山岩的地幔源区是被富钾硅酸盐熔体交代的陆下岩石圈地幔下部含金云母的石榴橄榄岩。结合非放射成因的铅成分，这些富钾硅酸盐熔体最有可能的来源是古老的俯冲大陆沉积物（> 1.5 Ga）。这些古老的俯冲沉积物具有相对较低的Pb初始同位素组成，在俯冲过程中经历了较大的U/Pb分馏，导致低μ（238 U/204 Pb），然后聚集在地幔过渡带中。这些超钾质岩石的87 Sr/86 Sr比值较低，说明它们的地幔源区具有较低的Rb/Sr比值，这可能是由古老俯冲沉积物的交代熔体造成的。这种解释是完全不同的，从以前的假设，属性其不寻常的地球化学特征，以一个占主导地位的软流圈源与分层的古SCLM的贡献，或SCLM源已被来自深软流圈或分层的古下陆壳的熔体交代。
The unique eruptions of ultrapotassic volcanic rocks in eastern China reported so far took place in the Xiaogulihe area of western Heilongjiang Province, NE China. These ultrapotassic rocks are characterized by extremely high K2O contents (> 7 wt.%), abnormally unradiogenic Pb isotopic compositions (206Pb/204Pb = 16.44–16.55;207Pb/204Pb = 15.39–15.46;208Pb/204Pb = 36.35–36.61), and moderately high87Sr/86Sr ratios (0.7053–0.7057), which can be basically correlated with those of ultrapotassic igneous rocks distributed widely in northwestern America and Aldan Shield. The positive correlation between187Os/188Os and 1/Os argues that these ultrapotassic rocks have probably experienced negligible lower continental crust addition (less than 1%) during magma ascent. The high contents of K2O and negative correlation between87Sr/86Sr and206Pb/204Pb of these ultrapotassic rocks indicate the presence of a potassic phase, mostly phlogopite, in their mantle source. Their strong fractionation of rare earth elements and lack of Nd–Hf isotopic decoupling reveal a low-degree partial melting of garnet-bearing source rocks. In addition, the low CaO and Al2O3contents of whole-rock compositions and low Fe/Mn ratios of olivine phenocryst chemistries suggest peridotites rather than pyroxenites as dominant source rocks for the Xiaogulihe ultrapotassic rocks. Based on these distinctive geochemical characteristics, we thus propose that the ultimate mantle source of the Xiaogulihe ultrapotassic volcanic rocks is phlogopite-bearing garnet peridotite within the lower part of the sub-continental lithospheric mantle (SCLM) that had been metasomatized by potassium-rich silicate melts. Combined with the unradiogenic Pb compositions, the most likely source of these potassium-rich silicate melts is the ancient subducted continental-derived sediments (> 1.5 Ga). These ancient subducted sediments, possessing relatively low initial Pb isotopic compositions, had experienced large U/Pb fractionation during a subduction process, resulting in low-μ (238U/204Pb), and then accumulated in the mantle transition zone. The relatively low87Sr/86Sr ratios of these ultrapotassic rocks also imply that their mantle source had evolved with low Rb/Sr ratios, which possibly resulted from the metasomatized melts derived from the ancient subducted sediments. This interpretation is quite different from previous hypotheses that attribute their unusual geochemical features to a dominantly asthenospheric source with a contribution from delaminated ancient SCLM, or a SCLM source that has been metasomatized by melts derived from deep asthenosphere or delaminated ancient lower continental crust.