In the manner or style of an antiquarian; with the careful, scholarly attention to old things and historical detail.
From antiquarian plus the adverbial suffix -ly. This follows standard English patterns for converting adjectives describing scholarly approaches into adverbs describing how something is done.
Adverbs ending in -ly often feel invisible, but this one reveals something: when we want to describe exactly HOW someone approaches the past, we need special words. It's the difference between looking at history and *studying* history.
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