馬里亞納海槽玄武巖中斜長(zhǎng)石礦物化學(xué)及意義

張平陽(yáng),鄢全樹,3*

(1.國(guó)家海洋局第一海洋研究所,山東青島266061;2.海洋沉積與環(huán)境地質(zhì)國(guó)家海洋局重點(diǎn)實(shí)驗(yàn)室,山東青島266061; 3.青島海洋科學(xué)與技術(shù)國(guó)家實(shí)驗(yàn)室海洋地質(zhì)過(guò)程與環(huán)境功能實(shí)驗(yàn)室,山東青島266061)

馬里亞納海槽玄武巖中斜長(zhǎng)石礦物化學(xué)及意義

張平陽(yáng)1,2,鄢全樹1,2,3*

(1.國(guó)家海洋局第一海洋研究所,山東青島266061;2.海洋沉積與環(huán)境地質(zhì)國(guó)家海洋局重點(diǎn)實(shí)驗(yàn)室,山東青島266061; 3.青島海洋科學(xué)與技術(shù)國(guó)家實(shí)驗(yàn)室海洋地質(zhì)過(guò)程與環(huán)境功能實(shí)驗(yàn)室,山東青島266061)

玄武巖中的斜長(zhǎng)石礦物記錄了巖漿過(guò)程的重要信息,可為揭示巖漿活動(dòng)規(guī)律提供重要線索。馬里亞納海槽玄武巖中斜長(zhǎng)石礦物具有兩種產(chǎn)出類型:斑晶和微晶。巖相學(xué)特征指示,斜長(zhǎng)石斑晶常具有聚片雙晶結(jié)構(gòu)、環(huán)帶結(jié)構(gòu),偶見(jiàn)熔蝕結(jié)構(gòu)和冷凝邊;斜長(zhǎng)石微晶呈半定向或雜亂分布在火山玻璃中,其中空骸晶結(jié)構(gòu)表明斜長(zhǎng)石微晶石在淬冷的條件下迅速形成。海槽18°N附近不同玄武巖樣品中環(huán)帶斜長(zhǎng)石斑晶環(huán)帶數(shù)與其寄主巖石Mg#值呈負(fù)相關(guān),指示了在馬里亞納海槽擴(kuò)張中心軸部地塹很小范圍內(nèi)獲得的玄武巖樣品經(jīng)歷的巖漿滯留時(shí)間以及巖漿混合作用周期存在一定程度的差異。海槽18°N,17°N和15°30'N附近3個(gè)位置的斜長(zhǎng)石斑晶邊部及斜長(zhǎng)石微晶的An值接近,表明這3個(gè)位置的巖漿活動(dòng)具有相似的巖漿上涌噴發(fā)通道結(jié)構(gòu)及海底淬冷環(huán)境。與沖繩海槽相比,馬里亞納海槽玄武巖具有更深的產(chǎn)出水深;同時(shí),研究區(qū)斜長(zhǎng)石微晶An值總體上略高于前兩者,這可能反映了水深對(duì)斜長(zhǎng)石微晶An值的影響。研究區(qū)斜長(zhǎng)石斑晶的形成溫度范圍比沖繩海槽更窄,暗示與沖繩海槽(陸緣弧后盆地)相比,馬里亞納海槽(洋內(nèi)弧后盆地)巖漿過(guò)程所經(jīng)歷的物理化學(xué)環(huán)境可能相對(duì)更穩(wěn)定。此外,微晶斜長(zhǎng)石結(jié)晶溫度略高于沖繩海槽,反映出兩個(gè)弧后擴(kuò)張區(qū)域巖漿噴出淬冷結(jié)晶時(shí)的水深的差異。

玄武質(zhì)巖石;斜長(zhǎng)石;礦物化學(xué);結(jié)晶溫度;馬里亞納海槽

斜長(zhǎng)石是玄武巖中主要的造巖礦物,其化學(xué)成分變化可能記錄了巖漿形成、上升運(yùn)移直至噴出地表固結(jié)成巖等全部巖漿過(guò)程中的重要信息。斜長(zhǎng)石晶體在結(jié)晶過(guò)程中可類質(zhì)同象相互替代的陽(yáng)離子(Na+)(Si4+) -(Ca2+)(Al3+)之間的低擴(kuò)散速率使得不同環(huán)帶之間的信息得以很好的保留[1]。前人已開展了斜長(zhǎng)石晶體隱含的巖漿所經(jīng)歷的物理化學(xué)條件等方面的諸多研究,如巖漿的混合[2]、巖漿上升過(guò)程中晶體發(fā)生重吸收[3]、巖漿房的分層作用[4-5]、巖漿房對(duì)流[6]以及結(jié)晶條件[6-14]等。近年來(lái)利用海底玄武巖中斜長(zhǎng)石的礦物化學(xué)來(lái)研究巖漿過(guò)程主要集中在島弧玄武巖[15-16]、大洋中脊玄武巖[18-20]等,而針對(duì)弧后區(qū)域玄武巖開展系統(tǒng)的斜長(zhǎng)石礦物化學(xué)研究不多[14,21-22]。馬里亞納海槽處于與沖繩海槽相似的弧后擴(kuò)張構(gòu)造背景之下。與沖繩海槽不同,馬里亞納海槽屬于相對(duì)簡(jiǎn)單的洋內(nèi)邊緣海盆,幾乎不受大陸地殼及陸源物質(zhì)的影響。近年來(lái)對(duì)馬里亞納海槽巖漿活動(dòng)的研究通常將馬里亞納海槽與其東側(cè)的馬里亞納火山弧作為一個(gè)整體,主要利用馬里亞納弧及弧后區(qū)域獲得的玄武巖進(jìn)行巖石地球化學(xué)研究,揭示與俯沖相關(guān)的巖漿活動(dòng)特征、地幔源區(qū)性質(zhì)及俯沖組分對(duì)此系統(tǒng)巖漿過(guò)程的影響[23-27],然而對(duì)馬里亞納海槽玄武巖單礦物的礦物化學(xué)研究工作甚少[28]。在細(xì)致的巖相學(xué)工作基礎(chǔ)之上,利用電子探針?lè)治鰷y(cè)試技術(shù)獲得了大量馬里亞納海槽擴(kuò)張中心的玄武質(zhì)巖石中的斜長(zhǎng)石礦物化學(xué)數(shù)據(jù),并進(jìn)行了玄武巖成巖溫度的研究,探討了本區(qū)玄武巖巖漿過(guò)程,為更深入了解馬里亞納海槽擴(kuò)張中心的巖漿活動(dòng)提供證據(jù)。

(陳 靖 編輯)

圖1 馬里亞納海槽巖石取樣位置Fig.1 Sampling locations for basaltic rocks from the Mariana Trough

1 地質(zhì)背景及樣品描述

馬里亞納海槽是菲律賓海板塊4個(gè)弧后盆地(其他3個(gè)為西菲律賓海盆、四國(guó)海盆和帕里西維拉海盆)中面積最小、時(shí)代最年輕的海盆[28]。海槽東西最大寬度約為250 km(143°00'～145°30'E,18°N附近),平均寬100～120 km,南北長(zhǎng)約1 200 km,呈向東突出的新月形(圖1)。海槽東部與馬里亞納火山弧相鄰,西部與不活動(dòng)的西馬里亞納殘留脊(因馬里亞納海槽打開而終止活動(dòng)并殘留下來(lái))相鄰,海槽向北變窄,海槽最北端為西馬里亞納殘留脊和馬里亞納現(xiàn)代活動(dòng)火山弧交匯處(143°E,24°N附近),向南終止于接近馬里亞納海溝挑戰(zhàn)者深淵(Challenger's Deep)(142°35'30″E,11°22'24″N)附近的位置[23]。馬里亞納海槽水深變化在2 000～5 000 m,一般水深為3 500～4 500 m[29]。馬里亞納海槽正處于活躍的擴(kuò)張階段,擴(kuò)張自南向北,弧后擴(kuò)張大約起始于8 Ma,在約5 Ma左右形成了海底擴(kuò)張[30]。海槽擴(kuò)張中心區(qū)域具有與慢速擴(kuò)張中脊區(qū)域洋殼相似的結(jié)構(gòu),地震剖面顯示弧后地殼厚度為5～7 km[31]。此外,一些研究顯示海槽地殼結(jié)構(gòu)具有極其不均一的特征:海槽洋殼沿著擴(kuò)張軸走向向北(21°N以北)以及遠(yuǎn)離18°N附近的擴(kuò)張軸向西增厚[32-33]。從擴(kuò)張時(shí)代上看,馬里亞納海槽屬于西太平洋地區(qū)邊緣海盆3個(gè)擴(kuò)張幕中最晚的1個(gè)擴(kuò)張幕:晚中新世-第四紀(jì),同一擴(kuò)張幕的邊緣海盆還有沖繩海槽、馬里亞納海槽、北斐濟(jì)、勞海盆、哈維海槽及伍德拉克海等[34-35]。Martinez等[36]沿海槽擴(kuò)張中心走向,將擴(kuò)張軸分為4部分:1)北火山-構(gòu)造活動(dòng)帶(22°06'～24°00'N);2)南火山-構(gòu)造活動(dòng)帶(21°00'～22°06'N);3)中央地塹(19°42'～21°00'N);4)慢速海底擴(kuò)張區(qū)(15°00'～19°42'N)。其中前兩部分處于增進(jìn)裂解階段,可能同時(shí)“捕獲”了島弧及弧后巖漿作用;后兩部分顯示了良好的海底擴(kuò)張樣式[23,34]。而海槽13°10'N以南擴(kuò)張中心類似于快速擴(kuò)張脊,其巖漿供給可能受靠近島弧因素的影響而增強(qiáng)[37-38]。因此海槽南北擴(kuò)張速率有著較大差異,半擴(kuò)張速率在16°～18°N區(qū)域估計(jì)為1.5～2.2 cm/a[28-29,39]。

DSDP(深海鉆探計(jì)劃)、ODP(大洋鉆探計(jì)劃)、IODP(綜合大洋鉆探計(jì)劃)計(jì)劃實(shí)施以來(lái),涉及馬里亞納海槽區(qū)域的航次調(diào)查獲取了大量的沉積物和基巖樣品,如DSDP 60航次近垂直于馬里亞納弧—槽系統(tǒng)進(jìn)行了多個(gè)孔位的鉆探研究[34,39]。本研究的玄武巖樣品來(lái)自1988-07—08中國(guó)-聯(lián)邦德國(guó)馬里亞納和西菲律賓海盆海洋地質(zhì)聯(lián)合調(diào)查(簡(jiǎn)稱“馬里亞納Ⅰ”計(jì)劃)以及1990-07—08的中德合作“太陽(yáng)”號(hào)第69航次(簡(jiǎn)稱馬里亞納Ⅱ計(jì)劃)。在這2個(gè)航次調(diào)查期間,利用拖網(wǎng)(DS)和電視抓斗(TVG)分別在海槽擴(kuò)張中心18°N, 17°N,15°30'N附近獲取了豐富的、新鮮玄武質(zhì)巖石和玻璃樣品(圖1)[40-43],涉及的玄武質(zhì)巖石塊狀樣品取樣位置經(jīng)緯度及水深見(jiàn)圖1和表1。其中,18°N附近的樣品采集于由Hussong和Freyer[44]辨識(shí)出的18°N附近(17°30'～18°20'N)弧后擴(kuò)張中心的軸向地塹(圖1b)。

圖2 馬里亞納海槽玄武巖代表性樣品的斜長(zhǎng)石的斑晶、微晶鏡下特征(均為正交偏光)Fig.2 Petrographic characteristics of plagioclasephenocrysts and microlites in representative samples from the Mariana Trough(images obtained by polarizing microscope)

本研究所涉及的馬里亞納海槽玄武巖樣品的蝕變程度都很低,總體特征為灰黑色,斑狀結(jié)構(gòu),氣孔構(gòu)造。鏡下觀察到斑晶礦物主要有斜長(zhǎng)石、單斜輝石、橄欖石,斑晶質(zhì)量分?jǐn)?shù)為15%～35%,其中斜長(zhǎng)石斑晶約占斑晶總量的70%以上。各樣品之間,斑晶礦物在含量、每種斑晶所占比例及斑晶形態(tài)上稍有差別?；|(zhì)主要為火山玻璃和斜長(zhǎng)石、單斜輝石、橄欖石微晶等,形成間粒-間隱結(jié)構(gòu)。副礦物有磁鐵礦、尖晶石(很少) (圖2)。斜長(zhǎng)石斑晶根據(jù)粒徑大小可分為大小兩組:大者0.3 mm×1 mm～2 mm×6 mm,小者0.1 mm× 0.4 mm～0.2 mm×0.8 mm。斜長(zhǎng)石斑晶自形,具有聚片雙晶結(jié)構(gòu)、環(huán)帶結(jié)構(gòu),環(huán)帶的稀疏-致密程度在樣品之間有一定差異;常見(jiàn)各種熔蝕結(jié)構(gòu)和冷凝邊,系巖漿上升,壓力降低時(shí)發(fā)生熔蝕作用并在驟冷的條件下形成(圖2a)。更大的斜長(zhǎng)石斑晶通常包含更多的熔體和流體包裹體(如圖2c)。斜長(zhǎng)石微晶呈板條狀、針狀,長(zhǎng)0.1～0.5 mm,橫截面近方形,邊長(zhǎng)0.01～0.03 mm,雜亂分布在火山玻璃中,有時(shí)基質(zhì)斜長(zhǎng)石微晶在小范圍(如氣孔周圍、表層玄武玻璃內(nèi))內(nèi)呈定向排列。許多斜長(zhǎng)石微晶可見(jiàn)中空骸晶結(jié)構(gòu),表明這樣的斜長(zhǎng)石微晶石在淬冷條件下迅速形成。

3 結(jié)果及討論

斜長(zhǎng)石礦物的電子探針成分分析是在國(guó)家海洋局第一海洋研究所海洋沉積與環(huán)境地質(zhì)實(shí)驗(yàn)室的日本JEOL公司JXA-8230型電子探針?lè)治鰞x上完成的。儀器工作條件:加速電壓15 k V,電子束流約2×10-8A,電子束斑1μm,定量分析檢出限約100×10-6。采用鉀長(zhǎng)石、鈉長(zhǎng)石、鈣長(zhǎng)石及氧化物組合標(biāo)樣,ZAF法校正。本次分析RSD介于±2%。圖像分析采用背散射電子圖像及二次電子圖像。斜長(zhǎng)石斑晶和微晶化學(xué)成分分類圖見(jiàn)圖3。圖3a中樣品號(hào)為13DS2,14DS,14DS1,20DS1,26DS,32DS,43DS3,43DS4,30DS2;圖3b中樣品號(hào)為71GTV;圖3c中樣品號(hào)為89DS。斜長(zhǎng)石寄主巖石全巖主量元素的測(cè)試工作在冶金地質(zhì)山東局測(cè)試中心利用ICP-MS完成(另文詳細(xì)研究),本文所用到的部分主量數(shù)據(jù)列于表1,電子探針?lè)治鼋Y(jié)果列于表2。

表1 馬里亞納海槽玄武巖樣品位置及部分全巖主量元素?cái)?shù)據(jù)Table 1 Information for sampling locations and part of whole-rock major oxides compositions of basaltic rocks or glasses

續(xù)表

3.1 斜長(zhǎng)石斑晶化學(xué)成分及意義

海槽18°N附近玄武巖中斜長(zhǎng)石斑晶An值53～88,平均值76;17°N附近斜長(zhǎng)石斑晶An值66～85,平均值76;15°30'N附近斜長(zhǎng)石斑晶An值57～80,平均值73,屬于拉長(zhǎng)石-培長(zhǎng)石,以培長(zhǎng)石為主,少數(shù)斑晶邊緣為拉長(zhǎng)石(圖3)。斑晶邊緣的An值稍低或者接近于微晶斜長(zhǎng)石成分,典型如:20DS1樣品中斜長(zhǎng)石斑晶20DS1-pl-01、20DS1-pl-02邊緣An值54.2和53(表3)稍低于微晶斜長(zhǎng)石An值平均值59(表2);32DS樣品中斜長(zhǎng)石斑晶32DS-pl-10邊緣An值68.1(表3)接近微晶斜長(zhǎng)石An值平均值65(表2)。

圖3 斜長(zhǎng)石化學(xué)成分分類圖Fig.3 Plots of classifications of plagioclase species in terms of their compositions

張國(guó)良等[20]認(rèn)為小范圍內(nèi)獲得的不同分異程度的玄武巖表明巖漿房?jī)?nèi)同時(shí)存在巖漿成分的多樣性,也并不意味著強(qiáng)烈?guī)r漿混合作用。然而,如果玄武巖中斜長(zhǎng)石斑晶具有多個(gè)反環(huán)帶,則說(shuō)明巖漿經(jīng)歷了周期性的混合作用。玄武巖Mg#一般認(rèn)為代表巖漿的演化分異程度,相對(duì)于高M(jìn)g#巖漿,低Mg#巖漿一般經(jīng)歷了更長(zhǎng)的巖漿滯留時(shí)間。本次研究樣品中背散射圖像觀察到的兩個(gè)粒徑相近的具有環(huán)帶結(jié)構(gòu)的斜長(zhǎng)石斑晶,低Mg#值樣品20DS1(Mg#=0.53)中的環(huán)帶斜長(zhǎng)石斑晶20DS1-pl-01(圖4a)較之Mg#值稍高的樣品32DS (Mg#=0.6)中的環(huán)帶斜長(zhǎng)石斑晶32DS-pl-10(圖4b),前者具有更多、更加密集的環(huán)帶結(jié)構(gòu)。同時(shí)利用電子探針對(duì)上述兩個(gè)斜長(zhǎng)石沿垂直環(huán)帶方向從核部到邊部,選取間距疏密相近的測(cè)點(diǎn)進(jìn)行測(cè)試獲得的An值波動(dòng)曲線,也顯示出20DS1-pl-01至少具有2個(gè)以上的反環(huán)帶結(jié)構(gòu),而32DS-pl-10可能至少存在一個(gè)以上(圖4)。另外,32DS樣品相比20DS1樣品中的環(huán)帶斜長(zhǎng)石斑晶具有An值更高的核部(表3),后者的核部An值與前者斑晶中間部位(圖4b測(cè)點(diǎn)3)An值相當(dāng),表明20DS1樣品相對(duì)于32DS樣品的環(huán)帶斜長(zhǎng)石核部形成于更加演化的巖漿環(huán)境。因此這2個(gè)環(huán)帶斜長(zhǎng)石斑晶剖面的An值波動(dòng)形式及變化范圍的差異,記錄了斜長(zhǎng)石斑晶在分離結(jié)晶時(shí)經(jīng)歷不同周期的巖漿混合作用;結(jié)合這兩個(gè)環(huán)帶斜長(zhǎng)石斑晶寄主巖石的Mg#值差異,共同指示馬里亞納海槽擴(kuò)張中心軸部地塹很小范圍獲得的玄武巖樣品經(jīng)歷的巖漿滯留時(shí)間以及巖漿混合作用的周期存在一定程度的差異。

本次研究中典型環(huán)帶結(jié)構(gòu)的斜長(zhǎng)石晶體最外層均為正環(huán)帶,即邊緣的An值驟降,如20DS1-pl-01邊緣An值由75→69→54,20DS1-pl-02邊緣An值由72→52,32DS-pl-10邊緣An值由78→68(表3),代表巖漿噴出時(shí)的快速冷卻階段;同時(shí)這2個(gè)玄武巖樣品中環(huán)帶斜長(zhǎng)石斑晶邊緣的An值變化范圍接近,表明其淬冷時(shí)所處物理化學(xué)環(huán)境接近。

圖4 典型環(huán)帶斜長(zhǎng)石斑晶背散射圖像(左)及從核到邊的An值波動(dòng)曲線(右)Fig.4 Backscattered Electron Images(BSEs)for typical zoned plagioclase phenocrysts(left)and the variational curves for An values(from core to rim,right)

3.2 斜長(zhǎng)石微晶化學(xué)成分及意義

海槽18°N附近樣品的斜長(zhǎng)石微晶An值為58～75,平均值為66;海槽17°附近斜長(zhǎng)石微晶An值為63～75,平均值69,海槽15°N附近斜長(zhǎng)石微晶An值63～70,平均值67,屬于拉長(zhǎng)石-培長(zhǎng)石,以拉長(zhǎng)石為主,樣品30DS2中斜長(zhǎng)石微晶An值達(dá)到培長(zhǎng)石(表2)。斜長(zhǎng)石斑晶與微晶的An值范圍有很大一部分重合。從采樣位置來(lái)看,海槽中這3個(gè)位置的斜長(zhǎng)石微晶An值基本相近,結(jié)合著3處樣品都具有相似的斜長(zhǎng)石斑晶熔蝕現(xiàn)象、熔蝕麻點(diǎn)以及前述斜長(zhǎng)石斑晶冷凝邊具有普遍接近的An值,共同說(shuō)明這3個(gè)位置噴發(fā)的玄武巖巖漿具有相似的巖漿上涌噴發(fā)通道結(jié)構(gòu)以及海底淬冷環(huán)境。

表3 典型環(huán)帶斜長(zhǎng)石斑晶成分Table3 Compositionsoftypicalzonedplagioclasephenocryst

與琉球島弧、沖繩海槽、南大西洋洋中脊、琉球島弧、東海陸架以及南海的玄武巖中的斜長(zhǎng)石An值對(duì)比,發(fā)現(xiàn)馬里亞納海槽玄武巖中斜長(zhǎng)石斑晶An變化范圍與沖繩海槽玄武巖中斜長(zhǎng)石斑晶An值范圍最為接近(表4)。斜長(zhǎng)石基質(zhì)微晶是巖漿噴發(fā)淬冷時(shí)的產(chǎn)物,因此其An值與噴出位置水深(可換算成靜水壓力)可能有一定關(guān)系,具體體現(xiàn)在假定溫度恒定的前提下,水深增大、水壓的升高會(huì)導(dǎo)致晶出的斜長(zhǎng)石An增大[44]。本研究獲得的馬里亞納海槽玄武巖樣品相對(duì)于其他區(qū)域具有更深的產(chǎn)出水深,與同為擴(kuò)張環(huán)境的沖繩海槽、南大西洋中脊相比,本區(qū)斜長(zhǎng)石微晶An值高于后兩者,這可能反映了水深對(duì)斜長(zhǎng)石微晶An值的影響。

表4 馬里亞納海槽與其他區(qū)域玄武巖中斜長(zhǎng)石An值對(duì)比Table 4 Comparison of An values of plagioclases in basaltic rocks from the Mariana Trough and other areas

3.3 斜長(zhǎng)石結(jié)晶溫度計(jì)算及其意義

巖漿發(fā)生部分分離結(jié)晶時(shí)的溫度和壓力是檢驗(yàn)巖漿運(yùn)移和流體演化模型的關(guān)鍵信息[12]。因?yàn)樾遍L(zhǎng)石的形成與其所處的物理化學(xué)環(huán)境(尤其是溫度、壓力)密切相關(guān),斜長(zhǎng)石+熔體溫度計(jì)是最早被應(yīng)用估計(jì)巖漿系統(tǒng)溫度的地質(zhì)溫度計(jì)之一,利用斜長(zhǎng)石-熔體成分對(duì)平衡溫度壓力的估算前人已經(jīng)提出了較多的公式和校正方法[7-13],本次研究為了方便與其他區(qū)域研究結(jié)果進(jìn)行對(duì)比,利用Kudo和Weill[7]的斜長(zhǎng)石溫度標(biāo)定公式對(duì)本區(qū)的斜長(zhǎng)石斑晶和微晶結(jié)晶溫度進(jìn)行計(jì)算。計(jì)算公式如下:

式中:T為熱力學(xué)溫度(K);λ=(XNa*XSi)/(XCa*XAl)熔體;σ=XAb/XAn。本研究中以上2個(gè)公式中涉及熔體成分均采用寄主巖石的全巖化學(xué)成分(表1)。

根據(jù)上述公式及說(shuō)明,本文利用電子探針?lè)治鰯?shù)據(jù)及寄主全巖數(shù)據(jù)對(duì)斜長(zhǎng)石斑晶及微晶的溫度分別進(jìn)行了計(jì)算,斜長(zhǎng)石斑晶用公式(1)計(jì)算,微晶用公式(2)計(jì)算。將斜長(zhǎng)石結(jié)晶溫度計(jì)算結(jié)果換算為攝氏度(℃),平均值列于表5。

值得注意的是該公式計(jì)算的斜長(zhǎng)石斑晶溫度并不能代表源區(qū)的實(shí)際溫度,而可能是演化之后巖漿的溫度,源區(qū)溫度應(yīng)高于本文所計(jì)算的溫度。先前研究已經(jīng)注意到熔體中水的含量對(duì)斜長(zhǎng)石-熔體平衡溫度估計(jì)的影響[13,45-46]。馬里亞納海槽弧后盆地玄武巖的含水量(結(jié)晶水范圍約為0.2%～2.0%,估計(jì)值引自[48])超過(guò)了正常大洋中脊玄武巖的含水量(約為0.21%,估計(jì)值引自[49])上限,但含水量依然顯著低于島弧玄武巖。因此我們認(rèn)為本文利用Kudo和Weill[7]的公式的計(jì)算結(jié)果與陳小明等[21]計(jì)算的沖繩海槽玄武巖中斜長(zhǎng)石結(jié)晶溫度可進(jìn)行直接比較,而與正常大洋中脊玄武巖以及島弧區(qū)玄武巖斜長(zhǎng)石計(jì)算結(jié)果對(duì)比時(shí),應(yīng)考慮巖漿含水量引起斜長(zhǎng)石結(jié)晶溫度的差異,具體表現(xiàn)為更高含水量的巖漿具有實(shí)際上更低的斜長(zhǎng)石結(jié)晶溫度。

表5 根據(jù)Kudo和Weill[7]標(biāo)定公式計(jì)算的斜長(zhǎng)石結(jié)晶溫度Table 5 Crystallization temperatures of plagioclases calculated by the formula of Kudo and Weill[7]

結(jié)合以上說(shuō)明,本次研究主要將馬里亞納海槽與沖繩海槽玄武巖中斜長(zhǎng)石結(jié)晶溫度特征進(jìn)行比較(表6)。馬里亞納海槽18°N附近玄武巖樣品斜長(zhǎng)石斑晶結(jié)晶溫度975～1 212℃(平均值1 077℃),17°N附近結(jié)晶溫度為984～1 020℃(平均值1 006℃),15°30'N附近結(jié)晶溫度為1 030～1 095℃(平均值1 068℃),明顯低于沖繩海槽玄武巖中斜長(zhǎng)石斑晶的結(jié)晶溫度1 214～1 280℃(平均值1 250℃)。馬里亞納海槽18° N附近玄武巖樣品斜長(zhǎng)石微晶結(jié)晶溫度979～1 104℃(平均值1 063℃),17°N附近結(jié)晶溫度為1 044～1 058℃(平均值1 052℃),15°30'N附近結(jié)晶溫度為1 081～1 091℃(平均值1 084℃),略高于沖繩海槽玄武巖斜長(zhǎng)石微晶結(jié)晶溫度范圍1 012～1 060℃,反映了這2個(gè)弧后擴(kuò)張區(qū)域的巖漿噴出淬冷結(jié)晶時(shí)的水深的差異。整體上,本區(qū)斜長(zhǎng)石(斑晶+微晶)結(jié)晶溫度范圍為975～1 212℃,斜長(zhǎng)石微晶與斑晶的形成溫度范圍極為接近,與沖繩海槽的整體斜長(zhǎng)石結(jié)晶溫度920～1 280℃相比范圍更窄,這暗示了,與處于陸緣背景的、初始弧后擴(kuò)張的沖繩海槽相比,馬里亞納海槽(洋內(nèi)弧后盆地)巖漿過(guò)程所經(jīng)歷的物理化學(xué)環(huán)境可能相對(duì)更穩(wěn)定。此外,本區(qū)斜長(zhǎng)石形成溫度類似于南大西洋慢速擴(kuò)張中脊玄武巖的斜長(zhǎng)石結(jié)晶溫度[50]。

表6 馬里亞納海槽、沖繩海槽及南大西洋中脊的玄武巖中斜長(zhǎng)石結(jié)晶溫度對(duì)比Table 6 Comparison of crystallization temperatures of plagioclases of basaltic rocks from the Mariana Trough, the Okinawa Trough,and the southern mid-Atlantic ridge

4 結(jié) 論

本文對(duì)馬里亞納海槽3個(gè)區(qū)域構(gòu)造位置的12個(gè)拖網(wǎng)站位的海底熔巖樣品內(nèi)的斜長(zhǎng)石礦物開展了詳細(xì)的礦物化學(xué)研究,獲得了如下新認(rèn)識(shí):

1)馬里亞納海槽玄武巖中斜長(zhǎng)石礦物具有2種產(chǎn)出類型:斑晶和微晶。斜長(zhǎng)石斑晶常具有聚片雙晶結(jié)構(gòu)、環(huán)帶結(jié)構(gòu)、偶見(jiàn)熔蝕結(jié)構(gòu)和冷凝邊;斜長(zhǎng)石微晶呈半定向或雜亂分布在火山玻璃中,其中空骸晶結(jié)構(gòu)表明斜長(zhǎng)石微晶石在淬冷的條件下迅速形成。

2)海槽18°N附近獲取的玄武巖樣品中環(huán)帶斜長(zhǎng)石斑晶環(huán)帶數(shù)與其寄主巖石Mg#值呈負(fù)相關(guān),指示在馬里亞納海槽擴(kuò)張中心軸部地塹的很小范圍內(nèi)獲得的玄武巖樣品在經(jīng)歷的巖漿滯留時(shí)間以及巖漿混合作用周期上存在一定程度的差異。

3)海槽18°N,17°N和15°30'N附近這3個(gè)位置的玄武巖中斜長(zhǎng)石斑晶邊部及斜長(zhǎng)石微晶的An值接近,表明這三個(gè)位置的巖漿活動(dòng)具有相似的巖漿上涌噴發(fā)通道結(jié)構(gòu)及海底淬冷環(huán)境。與沖繩海槽相比,馬里亞納海槽玄武巖具有更深的平均產(chǎn)出水深;同時(shí)本區(qū)斜長(zhǎng)石微晶An值總體上略高于前者,這可能反映了水深對(duì)斜長(zhǎng)石微晶An值的影響。

4)本區(qū)斜長(zhǎng)石斑晶的形成溫度低于沖繩海槽,斜長(zhǎng)石結(jié)晶溫度范圍整體上比沖繩海槽更窄,暗示與處于陸緣背景的、初始弧后擴(kuò)張的沖繩海槽相比,馬里亞納海槽(洋內(nèi)弧后盆地)巖漿過(guò)程所經(jīng)歷的物理化學(xué)環(huán)境可能相對(duì)更穩(wěn)定。微晶斜長(zhǎng)石結(jié)晶溫度略高于沖繩海槽,反映出兩個(gè)弧后擴(kuò)張區(qū)域巖漿噴出淬冷結(jié)晶時(shí)的水深的差異。

致謝:劉焱光和吳世迎研究員提供本研究的巖石樣品及修改意見(jiàn)。

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Compositions of Plagioclase Hosted by Basaltic Rocks From the Mariana Trough and Their Petrogenesis Significances

ZHANG Ping-yang1,2,YAN Quan-shu1,2,3
(1.The First Institute of Oceanography,SOA,Qingdao 266061,China; 2.Key Laboratory of Marine Sedimentology and Environmental Geology,SOA,Qingdao 266061,China; 3.Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology,Qingdao 266061,China)

Plagioclase hosted by basalts records important information for magmatic processes,and can provide clues for indicating the characteristics of magmatic activities.Plagioclase phenocrysts and microlitesare studied in the basalts from the Mariana Trough.Petrographic characteristics show that plagioclasephenocrysts generally show polysynthetic twin or zoning textures and some have the melting corrosion structure and chilled margins.Plagioclase microlitesare randomly distributed in groundmass,and the hollow skeletal crystal texture for some microlites imply that they were formed under rapid crystallization and quenching conditions.The numbers of oscillatory zoning bands of plagioclase phenocrystsamong different basaltic rock samples(dredged from Axial Garben of Mariana Trough at 18°N)are negatively correlated with the Mg#value of their host rocks,implying that magma form the Mariana Trough back-arc spreading center varies in periodicity of magma mingling and residence time in a small scale.For all samples from 18°N, 17°N,15°30'N at Mariana Trough,the An values for rims of plagioclase phenocrysts are nearly equal to those for plagioclase microlites,indicating that there aresimilar magma upwelling channels and magma eruption environments in these three locations.The water depth of the Mariana Trough is deeper than that of Okinawa Trough,which is consistent with the higher An value of plagioclase microlites from basaltsin the Mariana Trough.The calculated crystallizing temperature range of plagioclase phenocrysts and microlites of Mariana Trough arenarrowerthan that of the Okinawa Trough,implying that magmas from the Mariana Trough(15°30'～18°00'N)may evolve under a relatively more stable physicaland chemical condition compared to those from Okinawa Trough(a nascentback-arc basin developed in the continental margin).In addition,the crystallizing temperatures of plagioclase microlites in basaltic rocks from the Mariana trough is generally higher than that from Okinawa Trough,indicating that there are differences in magma eruption environments between these two troughs.

Basaltic rocks;plagioclase;mineral chemistry;crystallizing temperature;Mariana Trough

November 4,2016

P736

A

1671-6647(2017)02-0234-15

10.3969/j.issn.1671-6647.2017.02.008

2016-11-04

國(guó)家自然科學(xué)基金項(xiàng)目——海底巖石學(xué)(41322036)和亞洲大陸邊緣演化及環(huán)境效應(yīng)(U1606401);“全球變化與海氣相互作用”專項(xiàng)——西太平洋俯沖帶及弧后盆地體系(GASI-GEOGE-02);青島海洋科學(xué)與技術(shù)國(guó)家實(shí)驗(yàn)室“鰲山人才計(jì)劃”項(xiàng)目(2015ASTP-ES16);中央級(jí)公益性科研院所基本科研業(yè)務(wù)費(fèi)專項(xiàng)資金資助項(xiàng)目——“束星北”青年學(xué)者項(xiàng)目(2016S01);山東省泰山學(xué)者工程項(xiàng)目(2017-2021)

張平陽(yáng)(1991-),男,黑龍江黑河人,碩士研究生,主要從事俯沖帶巖石學(xué)方面研究.E-mail:zpyfio@163.com

*通訊作者:鄢全樹(1976-),男,江西廣豐人,特聘研究員,博士,主要從事海底巖漿活動(dòng)與構(gòu)造演化方面研究.

E-mail:yanquanshu@163.com