PIECEMEAL EMPLACEMENT (AND CONTAMINATION?) OF A CALEDONIAN PLUTON

The ~455 Ma Horta intrusive complex of the Helgeland Nappe Complex (Uppermost Allochthon, Norwegian Caledonides) is approximately tear-drop shaped and is ~7 km long and ~6 km wide. Principal rock types range from diorite in the central, eastern and southern parts of the complex, to alkali-feldspar syenite and quartz syenite in the west and north. The dioritic rocks are typically silica saturated, some monzodiorite and monzonite is silica undersaturated (nepheline-bearing), and syenites are silica saturated or oversaturated. Nd, Sr, O, and C isotope data indicate that these variations were caused by high-T assimilation of calc-silicate rocks followed by lower-T assimilation of silicate rocks/melts. Calc-silicate screens reacted with Horta magmas to form endoskarn; whereas screens of marble, quartzite, and quartz-rich migmatite did not react. Most outcrops contain both syenite and diorite, primarily emplaced as multiple NNW- to NE-oriented dikes/sheets. Consistent intrusive relationships were not observed, except that the youngest dikes are sparse, E-W oriented granites. The contact zone between syenitic and dioritic parts of the pluton is up to one km wide, with dioritic sheets in coarse syenitic host. Mingling is common here, as it is throughout the complex. Examples of mingling relationships include diorite in syenite and monzonite; monzonite in syenite, and syenite in syenite. Thus, even though diking is the common mode of emplacement, mingling occurred widely. The field and geochemical data indicate that Horta magmas intruded as numerous (100's or 1000's) of sheets and that these magmas interacted significantly with their host rocks. This poses a dilemma: If diking was the predominant emplacement mechanism, did individual magma batches contain sufficient heat, and remain molten long enough, to assimilate host rocks? If not, was assimilation at deeper levels in a larger magma body?