Formation of the paleoproterozoic calc-silicate and high-pressure mafic granulite in the Jiaobei Terrane, Eastern Shandong, China

Abstract

Jiaobei terrane contains low to high grade Paleoproterozoic metamorphosed sedimentary sequence deposited under littoral to neritic environments, belonging to part of the Jiao-Liao-Ji belt of the Eastern Block in the North China Craton. The clock wise P-T paths concluded from HP mafic and pelitic granulites indicate the closing and collisional process of the belt. This study focuses on the formation of calc-silicate that is closely associated with HP mafic granulite. On the basis of mineral assemblage evolution of the PT peudosention in SACFM system, critical metamorphic reactions, combined with calculation of Grt-Cpx thermometer, the precursor granulite is estimated having experienced at least 780 ∼ 850°C and 1.0 - 1. 1GPa metamorphic conditions, and retrograde to 400 ∼ 650°C and 0. 6 ∼0. 75GPa. Accordingly, a clock wise P-T path is concluded. Results show that the calc-silicate develops on the late stage of continent-continent collision during the exhumation process of the HP granulite, combined with Ca metasomatism at the same time. Establishment of the genetic relationship of calc-silicate and the mafic garnet granulite present the importance of petrological paragenetic association during the exhumation of collisional metamorphic terrane.

膠北地塊發育一套早元古代變質雜巖,主要由淺變質到中高級變質的濱海相至淺海相的沉積巖系列組成,屬于膠-遼-吉構造活動帶的一部分。其中具有順時針變質作用PT軌跡的高壓基性麻粒巖和高壓泥質麻粒巖指示了造成該帶閉合的碰撞構造過程。本文研究與高壓基性麻粒巖密切共生的鈣硅酸鹽巖。首先根據礦物反應關系研究,確定鈣硅酸鹽巖是由含石榴石的基性麻粒巖經鈣質交代形成的。進一步采用殘留單斜輝石-石榴石組合、退變礦物組合,并根據SACMF體系礦物組合演化的PT視剖面圖,計算得出鈣硅酸鹽巖經歷了早期溫壓條件為780～850℃、1.0～1.1GPa的變質作用和晚期溫壓條件為400～650℃、0.6～0.75GPa的退變質作用和交代作用,從而得出順時針的P-T軌跡。這一結果表明,鈣硅酸鹽巖形成于陸-陸碰撞過程的晚期階段,是高壓基性麻粒巖在折返過程中經歷退變質和Ca質交代聯合作用的結果。鈣硅酸鹽巖的形成機制及其與石榴石基性麻粒巖原巖的成因聯系的確立,豐富了對于碰撞變質地塊抬升過程及其巖石學效應的認識。