Zircon and REE-rich alkaline plutonic rocks intruded into the accretionary prism at the Cape Ashizuri, Shikoku Island, Japan

Abstract

Abstract:The Cape Ashizuri body is a small, 12 km 2 , Miocene plutonic complex having gabbroids, syenite, quartz syenite to syenogranite in its southern half and biotite monzogranite in its northern half, right next to the Shimanto accretionary sediments. Chemical analyses indicate that the granitoids have mostly meta-aluminous value of A/CNK<1.0, except for three monzogranite. The granitoids are rich in K 2 O, plotted in the shoshonite-high K fields. The granitoids are also rich in Na 2 O and Rb. Ga content is higher than 18 ppm, indicating characteristics of the A-type granitoids. A distinct characteristic of the Cape Ashizuri granitoids is predominance of HFSE (high-field-strength-element), such as Zr (<1,220 ppm), Hf (25.6 ppm), Nb (<202 ppm), Ta (<14 ppm), LREE (<1002 ppm), HREE (<50.5 ppm), Y (<74 ppm), Th (74.2 ppm). Fluorine is also enriched up to 0.43 wt.%.These HFSE are contained in such common accessory minerals as zircon, ilmenite, allanite and titanite, and rare minerals of fergusonite, chevkinite and others. Zircon is the most common and visible rarely by naked eyes. Zircon in the highest Zr rock, 58A142 sample, contains the maximum values of 2.4 wt.% HfO 2 and 1.0 wt.% Y 2 O 3 . Ilmenite in the 58A142 sample shows 4.3-5.7 wt.% MnO, but that in other quartz syenite shows lower values as 1.7-1.8 wt.% MnO, while its niobium content, up to 3.8 wt.% Nb 2 O 5 , is seen along the rim, indicating the niobium concentrated at the latest magmatic stage. Allanite shows Mn-poor character of the magnetite series (0.44 wt.% MnO), and contains 24.6-25.8 wt.% LREE. The most mafic phase of the gabbroids is 47.0 wt.% SiO 2 and 12.5 wt.% MgO, indicating its origin of the upper mantle. Quartz syenitic rocks have 55-60 wt.% SiO 2 and low Sr 0 ratio of 0.7035, implying a small degree of the partial melting of mafic igneous source rocks in the lower crust for the alkaline rocks. The high contents of HFSE and F were caused by this small degree of the partial melting. The biotite monzogranite in the northern part is considered as a mixed magma generated from the lower igneous crust and overlying accreted sediments of the Shimanto Supergroup. It is interesting to explore hydrothermal REE concentration related to these plutonic and subvolcanic activities.

Geochemical characteristics of zircons in the Ashizuri A‐type granitoids: An additional granite topology tool for detrital zircon studies

“…Except for the gabbro (SS65), granitoids show clear negative Eu anomalies, and their degree is the largest in stage II quartz syenite (SS67). In terms of these features concerning to the REE, our result is consistent with those in previous works (Ishihara & Hoshino, 2013;Ishihara & Murakami, 2006;Murakami & Masuda, 1984).…”

“…Along with the F-bearing minerals, these lines of evidences suggest that granitoids used in this study have general features of A-type granites proposed by Whalen et al Ashizuri plutons make a single trend in aluminous saturation index-SiO 2 diagram (Figure 4b), in which metaluminousness decreases toward the low silica rocks. This fact suggests that feldspars in the Ashizuri plutons underwent the same fractionation trend from gabbroids to the granitoids, as proposed in the previous work(Ishihara & Hoshino, 2013).…”

Oxygen Isotopic Compositions of the Cape Ashizuri Granitoids in Southwest Japan

“…Major chemistry, 2 of 10 from Ishihara and Murakami () and 8 from Ishihara and Hoshino (). δO, analyzed by Inst.…”

“…The general high δ 18 O values on the Outer Zone granitoids are considered due to mixing of deepcrustal origin of the granitic magmas generated in I-type source rocks and the fore-arc sedimentary rocks within the continental crust (Ishihara & Matsuhisa, 1999). The general low δ 18 O values on the biotite granite Major chemistry, 2 of 10 from Ishihara and Murakami (2006) and 8 from Ishihara and Hoshino (2013 of the studied area are considered due to less sedimentary rocks in the source region for this magma.…”

“…According to the best lithological map of T. Imaoka (Fig. 2), the sedimentary xenolithic blocks are described more in the biotite granite than in the syenites, and thus contamination effect of the sediments was considered for the genesis of the biotite granite (Ishihara & Hoshino, 2013). Contribution of the sediments into granitic magmas is best shown by whole-rock δ 18 O values, and relatively high δ 18 O values were recognized throughout the granitic bodies of the Outer Zone of Southwest Japan (Ishihara & Matsuhisa, 1999); yet no data have been available for the Cape Ashizuri plutonic body.…”